[clang] Require including config.h for CLANG_DEFAULT_STD_C

This makes CLANG_DEFAULT_STD_C(XX) always be defined, defaulting to
lang_unspecified, so you are forced to check its value instead of using
an #ifdef. This should help avoid accidentally omitting the include in
places where that's important, so that the default language version bug
isn't re-introduced.

Reviewed By: hokein, dexonsmith

Differential Revision: https://reviews.llvm.org/D124974

clang/include/clang/Config/config.h.cmake

@@ -16,9 +16,21 @@
 
 /* Default C/ObjC standard to use. */
 #cmakedefine CLANG_DEFAULT_STD_C LangStandard::lang_${CLANG_DEFAULT_STD_C}
+/* Always #define something so that missing the config.h #include at use sites
+ * becomes a compile error.
+ */
+#ifndef CLANG_DEFAULT_STD_C
+#define CLANG_DEFAULT_STD_C LangStandard::lang_unspecified
+#endif
 
 /* Default C++/ObjC++ standard to use. */
 #cmakedefine CLANG_DEFAULT_STD_CXX LangStandard::lang_${CLANG_DEFAULT_STD_CXX}
+/* Always #define something so that missing the config.h #include at use sites
+ * becomes a compile error.
+ */
+#ifndef CLANG_DEFAULT_STD_CXX
+#define CLANG_DEFAULT_STD_CXX LangStandard::lang_unspecified
+#endif
 
 /* Default C++ stdlib to use. */
 #define CLANG_DEFAULT_CXX_STDLIB "${CLANG_DEFAULT_CXX_STDLIB}"

clang/lib/Basic/LangStandards.cpp

@@ -58,30 +58,27 @@ LangStandard::Kind clang::getDefaultLanguageStandard(clang::Language Lang,
     return LangStandard::lang_cuda;
   case Language::Asm:
   case Language::C:
-#if defined(CLANG_DEFAULT_STD_C)
-    return CLANG_DEFAULT_STD_C;
-#else
+    if (CLANG_DEFAULT_STD_C != LangStandard::lang_unspecified)
+      return CLANG_DEFAULT_STD_C;
+
     // The PS4 uses C99 as the default C standard.
     if (T.isPS4())
       return LangStandard::lang_gnu99;
     return LangStandard::lang_gnu17;
-#endif
   case Language::ObjC:
-#if defined(CLANG_DEFAULT_STD_C)
-    return CLANG_DEFAULT_STD_C;
-#else
+    if (CLANG_DEFAULT_STD_C != LangStandard::lang_unspecified)
+      return CLANG_DEFAULT_STD_C;
+
     return LangStandard::lang_gnu11;
-#endif
   case Language::CXX:
   case Language::ObjCXX:
-#if defined(CLANG_DEFAULT_STD_CXX)
-    return CLANG_DEFAULT_STD_CXX;
-#else
+    if (CLANG_DEFAULT_STD_CXX != LangStandard::lang_unspecified)
+      return CLANG_DEFAULT_STD_CXX;
+
     if (T.isDriverKit())
       return LangStandard::lang_gnucxx17;
     else
       return LangStandard::lang_gnucxx14;
-#endif
   case Language::RenderScript:
     return LangStandard::lang_c99;
   case Language::HIP: