Switch TestVisitor to Lang_C via -x c

[sberg]

Summary

...instead of -std=c99, as the latter lead to

error: invalid argument '-std=c99' not allowed with 'C++'

complaints in test logs

unittests/Tooling/TestVisitor.h

enum Language {
     Lang_OBJCXX11,
     Lang_CXX = Lang_CXX98
   };
 
   /// \brief Runs the current AST visitor over the given code.
   bool runOver(StringRef Code, Language L = Lang_CXX) {
     std::vector<std::string> Args;
     switch (L) {
-      case Lang_C: Args.push_back("-std=c99"); break;
+      case Lang_C:
+        Args.push_back("-x");
+        Args.push_back("c");
+        break;
       case Lang_CXX98: Args.push_back("-std=c++98"); break;
       case Lang_CXX11: Args.push_back("-std=c++11"); break;
       case Lang_CXX14: Args.push_back("-std=c++14"); break;
       case Lang_OBJC: Args.push_back("-ObjC"); break;
       case Lang_OBJCXX11:
         Args.push_back("-ObjC++");
         Args.push_back("-std=c++11");
         Args.push_back("-fblocks");