Add ability to test the translator without clang

[AlexeySachkov]

Right now we are using clang via:

SPIRV-LLVM-Translator/test/lit.cfg.py

Line 44 in a4a8f15

llvm_config.use_clang()

It emits error if clang wasn't found. However, it has an optional argument required, which can be set to False.

I have WIP branch, which switches this flags and adds REQUIRED directives to corresponding .cl tests here, but local testing shows, that required=False is not really supported: use_clang crashes somewhere withing it with exception