Reapply Allow system header to provide their own implementation of some builtin

This reverts commit 3d210ed3d1880c615776b07d1916edb400c245a6.

See https://reviews.llvm.org/D71082 for the patch and discussion that make it
possible to reapply this patch.
diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
index be59d88..0d30f64 100644
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -3046,6 +3046,14 @@
   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