Repair recognition of "CC" as an alias for the C++ compiler, since it

was silently broken by upstream for a Windows-specific use-case.

Apparently some versions of CMake still rely on this archaic feature...

Reported by:	rakuco
MFC after:	3 days
X-MFC-With:	r261991

contrib/llvm/tools/clang/tools/driver/driver.cpp

@@ -229,8 +229,10 @@ static void ParseProgName(SmallVectorImpl<const char *> &ArgVector,
     { "++",        "--driver-mode=g++" },
   };
   std::string ProgName(llvm::sys::path::stem(ArgVector[0]));
+#ifdef _WIN32
   std::transform(ProgName.begin(), ProgName.end(), ProgName.begin(),
                  toLowercase);
+#endif
   StringRef ProgNameRef(ProgName);
   StringRef Prefix;