Revert "Allow system header to provide their own implementation of some builtin"

This reverts commit 921f871 because it
causes libc++ code to trigger __warn_memset_zero_len.

See https://reviews.llvm.org/D71082.

Author: amykhuang

clang/include/clang/AST/Decl.h

@@ -2309,9 +2309,6 @@ class FunctionDecl : public DeclaratorDecl,
   /// true through IsAligned.
   bool isReplaceableGlobalAllocationFunction(bool *IsAligned = nullptr) const;
 
-  /// Determine if this function provides an inline implementation of a builtin.
-  bool isInlineBuiltinDeclaration() const;
-
   /// Determine whether this is a destroying operator delete.
   bool isDestroyingOperatorDelete() const;
 

clang/lib/AST/Decl.cpp

@@ -3046,14 +3046,6 @@ bool FunctionDecl::isReplaceableGlobalAllocationFunction(bool *IsAligned) const
   return Params == FPT->getNumParams();
 }
 
-bool FunctionDecl::isInlineBuiltinDeclaration() const {
-  if (!getBuiltinID())
-    return false;
-
-  const FunctionDecl *Definition;
-  return hasBody(Definition) && Definition->isInlineSpecified();
-}
-
 bool FunctionDecl::isDestroyingOperatorDelete() const {
   // C++ P0722:
   //   Within a class C, a single object deallocation function with signature

clang/lib/CodeGen/CGExpr.cpp

@@ -4621,15 +4621,8 @@ RValue CodeGenFunction::EmitSimpleCallExpr(const CallExpr *E,
 }
 
 static CGCallee EmitDirectCallee(CodeGenFunction &CGF, const FunctionDecl *FD) {
-
   if (auto builtinID = FD->getBuiltinID()) {
-    // Replaceable builtin provide their own implementation of a builtin. Unless
-    // we are in the builtin implementation itself, don't call the actual
-    // builtin. If we are in the builtin implementation, avoid trivial infinite
-    // recursion.
-    if (!FD->isInlineBuiltinDeclaration() ||
-        CGF.CurFn->getName() == FD->getName())
-      return CGCallee::forBuiltin(builtinID, FD);
+    return CGCallee::forBuiltin(builtinID, FD);
   }
 
   llvm::Constant *calleePtr = EmitFunctionDeclPointer(CGF.CGM, FD);

clang/lib/CodeGen/CodeGenModule.cpp

@@ -1840,11 +1840,6 @@ void CodeGenModule::SetFunctionAttributes(GlobalDecl GD, llvm::Function *F,
   else if (const auto *SA = FD->getAttr<SectionAttr>())
      F->setSection(SA->getName());
 
-  if (FD->isInlineBuiltinDeclaration()) {
-    F->addAttribute(llvm::AttributeList::FunctionIndex,
-                    llvm::Attribute::NoBuiltin);
-  }
-
   if (FD->isReplaceableGlobalAllocationFunction()) {
     // A replaceable global allocation function does not act like a builtin by
     // default, only if it is invoked by a new-expression or delete-expression.