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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@305044 91177308-0d34-0410-b5e6-96231b3b80d8