check-clang: Introduce the feature "utf8-capable-terminal".

clang/test/FixIt/fixit-unicode-with-utf8-output.c has begun complained since LLVM r200885.
Although it is changes for StringRef, it brought LLVM_ON_WIN32 to Support/Locale.cpp.

Before r200885, LLVM_ON_WIN32 was undefined in Locale.cpp!

FIXME: We should consider i18n on win32.