PointerLikeTypeTraits: Standardize NumLowBitsAvailable on static constexpr rather than anonymous enum

This is (more?) usable by GDB pretty printers and seems nicer to write.

There's one tricky caveat that in C++14 (LLVM's codebase today) the
static constexpr member declaration is not a definition - so odr use of
this constant requires an out of line definition, which won't be
provided (that'd make all these trait classes more annoyidng/expensive
to maintain). But the use of this constant in the library implementation
is/should always be in a non-odr context - only two unit tests needed to
be touched to cope with this/avoid odr using these constants.

Based on/expanded from D72590 by Christian Sigg.

Author: dwblaikie

clang/include/clang/AST/CanonicalType.h

@@ -384,7 +384,7 @@ struct PointerLikeTypeTraits<clang::CanQual<T>> {
   }
 
   // qualifier information is encoded in the low bits.
-  enum { NumLowBitsAvailable = 0 };
+  static constexpr int NumLowBitsAvailable = 0;
 };
 
 } // namespace llvm

clang/include/clang/AST/DeclGroup.h

@@ -147,7 +147,7 @@ namespace llvm {
       return clang::DeclGroupRef::getFromOpaquePtr(P);
     }
 
-    enum { NumLowBitsAvailable = 0 };
+    static constexpr int NumLowBitsAvailable = 0;
   };
 
 } // namespace llvm

clang/include/clang/AST/ExternalASTSource.h

@@ -504,9 +504,8 @@ struct PointerLikeTypeTraits<
   static void *getAsVoidPointer(Ptr P) { return P.getOpaqueValue(); }
   static Ptr getFromVoidPointer(void *P) { return Ptr::getFromOpaqueValue(P); }
 
-  enum {
-    NumLowBitsAvailable = PointerLikeTypeTraits<T>::NumLowBitsAvailable - 1
-  };
+  static constexpr int NumLowBitsAvailable =
+      PointerLikeTypeTraits<T>::NumLowBitsAvailable - 1;
 };
 
 } // namespace llvm

clang/include/clang/AST/TemplateName.h

@@ -559,7 +559,7 @@ struct PointerLikeTypeTraits<clang::TemplateName> {
   }
 
   // No bits are available!
-  enum { NumLowBitsAvailable = 0 };
+  static constexpr int NumLowBitsAvailable = 0;
 };
 
 } // namespace llvm.

clang/include/clang/AST/Type.h

@@ -85,7 +85,7 @@ namespace llvm {
       return static_cast< ::clang::Type*>(P);
     }
 
-    enum { NumLowBitsAvailable = clang::TypeAlignmentInBits };
+    static constexpr int NumLowBitsAvailable = clang::TypeAlignmentInBits;
   };
 
   template<>
@@ -96,7 +96,7 @@ namespace llvm {
       return static_cast< ::clang::ExtQuals*>(P);
     }
 
-    enum { NumLowBitsAvailable = clang::TypeAlignmentInBits };
+    static constexpr int NumLowBitsAvailable = clang::TypeAlignmentInBits;
   };
 
 } // namespace llvm
@@ -1295,7 +1295,7 @@ struct PointerLikeTypeTraits<clang::QualType> {
   }
 
   // Various qualifiers go in low bits.
-  enum { NumLowBitsAvailable = 0 };
+  static constexpr int NumLowBitsAvailable = 0;
 };
 
 } // namespace llvm

clang/include/clang/Basic/IdentifierTable.h

@@ -967,7 +967,7 @@ struct PointerLikeTypeTraits<clang::Selector> {
     return clang::Selector(reinterpret_cast<uintptr_t>(P));
   }
 
-  enum { NumLowBitsAvailable = 0 };
+  static constexpr int NumLowBitsAvailable = 0;
 };
 
 // Provide PointerLikeTypeTraits for IdentifierInfo pointers, which
@@ -982,7 +982,7 @@ struct PointerLikeTypeTraits<clang::IdentifierInfo*> {
     return static_cast<clang::IdentifierInfo*>(P);
   }
 
-  enum { NumLowBitsAvailable = 1 };
+  static constexpr int NumLowBitsAvailable = 1;
 };
 
 template<>
@@ -995,7 +995,7 @@ struct PointerLikeTypeTraits<const clang::IdentifierInfo*> {
     return static_cast<const clang::IdentifierInfo*>(P);
   }
 
-  enum { NumLowBitsAvailable = 1 };
+  static constexpr int NumLowBitsAvailable = 1;
 };
 
 } // namespace llvm

clang/include/clang/Basic/SourceLocation.h

@@ -482,7 +482,7 @@ namespace llvm {
   // Teach SmallPtrSet how to handle SourceLocation.
   template<>
   struct PointerLikeTypeTraits<clang::SourceLocation> {
-    enum { NumLowBitsAvailable = 0 };
+    static constexpr int NumLowBitsAvailable = 0;
 
     static void *getAsVoidPointer(clang::SourceLocation L) {
       return L.getPtrEncoding();

clang/include/clang/CodeGen/ConstantInitFuture.h

@@ -35,7 +35,7 @@ struct PointerLikeTypeTraits< ::clang::CodeGen::ConstantInitBuilderBase*> {
 
   static inline void *getAsVoidPointer(T p) { return p; }
   static inline T getFromVoidPointer(void *p) {return static_cast<T>(p);}
-  enum { NumLowBitsAvailable = 2 };
+  static constexpr int NumLowBitsAvailable = 2;
 };
 }
 
@@ -79,10 +79,8 @@ class ConstantInitFuture {
     result.Data = PairTy::getFromOpaqueValue(value);
     return result;
   }
-  enum {
-    NumLowBitsAvailable =
-      llvm::PointerLikeTypeTraits<PairTy>::NumLowBitsAvailable
-  };
+  static constexpr int NumLowBitsAvailable =
+      llvm::PointerLikeTypeTraits<PairTy>::NumLowBitsAvailable;
 };
 
 }  // end namespace CodeGen
@@ -100,7 +98,7 @@ struct PointerLikeTypeTraits< ::clang::CodeGen::ConstantInitFuture> {
   static inline T getFromVoidPointer(void *p) {
     return T::getFromOpaqueValue(p);
   }
-  enum { NumLowBitsAvailable = T::NumLowBitsAvailable };
+  static constexpr int NumLowBitsAvailable = T::NumLowBitsAvailable;
 };
 
 } // end namespace llvm

clang/include/clang/Sema/Ownership.h

@@ -116,7 +116,7 @@ namespace llvm {
 
   template <class T>
   struct PointerLikeTypeTraits<clang::OpaquePtr<T>> {
-    enum { NumLowBitsAvailable = 0 };
+    static constexpr int NumLowBitsAvailable = 0;
 
     static inline void *getAsVoidPointer(clang::OpaquePtr<T> P) {
       // FIXME: Doesn't work? return P.getAs< void >();

llvm/include/llvm/ADT/IntervalMap.h

@@ -491,7 +491,7 @@ class NodeRef {
   struct CacheAlignedPointerTraits {
     static inline void *getAsVoidPointer(void *P) { return P; }
     static inline void *getFromVoidPointer(void *P) { return P; }
-    enum { NumLowBitsAvailable = Log2CacheLine };
+    static constexpr int NumLowBitsAvailable = Log2CacheLine;
   };
   PointerIntPair<void*, Log2CacheLine, unsigned, CacheAlignedPointerTraits> pip;