[libclang] Introduce a new parsing option 'CXTranslationUnit_SingleFileParse'...

[libclang] Introduce a new parsing option 'CXTranslationUnit_SingleFileParse' that puts preprocessor in a mode for parsing a single file only.

This is useful for parsing a single file, as a fast/inaccurate 'mode' that can still provide declarations from the file, like the classes and their methods.

llvm-svn: 305044