[APINotes] Preserve attributes from inactive versions.

...by wrapping them in another (new) attribute, SwiftVersionedAttr.
This can track both attributes that would be added and those that
would be removed.

This doesn't support other kinds of changes that can be made via API
notes, such as nullability or wholesale type changes, but it's a place
to start, and possibly sufficient for our goals.

Part of rdar://problem/28618121.

Author: jrose-apple

include/clang/AST/Attr.h

@@ -24,6 +24,7 @@
 #include "clang/Basic/Sanitizers.h"
 #include "clang/Basic/SourceLocation.h"
 #include "clang/Basic/VersionTuple.h"
+#include "llvm/ADT/PointerEmbeddedInt.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/raw_ostream.h"

include/clang/Basic/Attr.td

@@ -1483,6 +1483,44 @@ def SwiftImportPropertyAsAccessors : InheritableAttr {
   let Documentation = [Undocumented];
 }
 
+def SwiftVersioned : Attr {
+  // This attribute has no spellings as it is only ever created implicitly
+  // from API notes.
+  let Spellings = [];
+  let Args = [VersionArgument<"Version">];
+  let SemaHandler = 0;
+  let Documentation = [Undocumented];
+  let AdditionalMembers = [{
+  private:
+    llvm::PointerUnion<const Attr *,
+                       llvm::PointerEmbeddedInt<attr::Kind>> attrOrKind;
+    bool hasBeenSet() const {
+      return !attrOrKind.is<const Attr *>() || !attrOrKind.isNull();
+    }
+  public:
+    bool isRemoval() const {
+      assert(hasBeenSet());
+      return !attrOrKind.is<const Attr *>();
+    }
+    const Attr *getAttrToAdd() const {
+      assert(hasBeenSet());
+      return attrOrKind.get<const Attr *>();
+    }
+    attr::Kind getAttrToRemove() const {
+      assert(hasBeenSet());
+      return attrOrKind.get<llvm::PointerEmbeddedInt<attr::Kind>>();
+    }
+    void setAttrToAdd(const Attr *attr) {
+      assert(!hasBeenSet());
+      attrOrKind = attr;
+    }
+    void setAttrToRemove(attr::Kind kind) {
+      assert(!hasBeenSet());
+      attrOrKind = kind;
+    }
+  }];
+}
+
 def ReqdWorkGroupSize : InheritableAttr {
   let Spellings = [GNU<"reqd_work_group_size">];
   let Args = [UnsignedArgument<"XDim">, UnsignedArgument<"YDim">,