[TBD] Validate TBDs in debug compilers for all platforms.

This is generalizing caa3dd4 to work even on
non-Apple platforms.

Fixes rdar://problem/40431434.

Author: huonw

lib/FrontendTool/FrontendTool.cpp

@@ -1171,15 +1171,13 @@ static bool validateTBDIfNeeded(CompilerInvocation &Invocation,
   switch (mode) {
   case FrontendOptions::TBDValidationMode::Default:
 #ifndef NDEBUG
-    // When a debug compiler is targeting an apple platform, we do some
-    // validation by default.
-    if (Invocation.getLangOptions().Target.getVendor() == llvm::Triple::Apple) {
-      mode = FrontendOptions::TBDValidationMode::MissingFromTBD;
-      break;
-    }
-#endif
+    // With a debug compiler, we do some validation by default.
+    mode = FrontendOptions::TBDValidationMode::MissingFromTBD;
+    break;
+#else
     // Otherwise, the default is to do nothing.
     LLVM_FALLTHROUGH;
+#endif
   case FrontendOptions::TBDValidationMode::None:
     return false;
   case FrontendOptions::TBDValidationMode::All: