Disallow use of __has_c_attribute in C++ mode.

__has_cpp_attribute is not available in C mode, and __has_c_attribute
should not be available in C++ mode. This also adds a test to
demonstrate that we properly handle scoped attribute tokens even in C
mode.

clang/lib/Lex/PPMacroExpansion.cpp

@@ -370,7 +370,11 @@ void Preprocessor::RegisterBuiltinMacros() {
   Ident__has_extension    = RegisterBuiltinMacro(*this, "__has_extension");
   Ident__has_builtin      = RegisterBuiltinMacro(*this, "__has_builtin");
   Ident__has_attribute    = RegisterBuiltinMacro(*this, "__has_attribute");
-  Ident__has_c_attribute  = RegisterBuiltinMacro(*this, "__has_c_attribute");
+  if (!LangOpts.CPlusPlus)
+    Ident__has_c_attribute = RegisterBuiltinMacro(*this, "__has_c_attribute");
+  else
+    Ident__has_c_attribute = nullptr;
+
   Ident__has_declspec = RegisterBuiltinMacro(*this, "__has_declspec_attribute");
   Ident__has_include      = RegisterBuiltinMacro(*this, "__has_include");
   Ident__has_include_next = RegisterBuiltinMacro(*this, "__has_include_next");

clang/test/Preprocessor/has_c_attribute.c

@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -fdouble-square-bracket-attributes -std=c11 -E %s -o - | FileCheck %s
+// RUN: %clang_cc1 -std=c2x -E %s -o - | FileCheck %s
 
 // CHECK: has_fallthrough
 #if __has_c_attribute(fallthrough)
@@ -14,3 +15,8 @@
 #if __has_c_attribute(__nodiscard__)
   int has_nodiscard_underscore();
 #endif
+
+// CHECK: has_clang_annotate
+#if __has_c_attribute(clang::annotate)
+  int has_clang_annotate();
+#endif

clang/test/Preprocessor/has_c_attribute.cpp

@@ -0,0 +1,8 @@
+// RUN: %clang_cc1 -std=c++11 %s -verify
+
+#if __has_c_attribute(fallthrough) // expected-error {{function-like macro '__has_c_attribute' is not defined}}
+#endif
+
+#if __has_c_attribute(gnu::transparent_union) // expected-error {{function-like macro '__has_c_attribute' is not defined}}
+#endif
+