[libclang] Add clang_getAllSkippedRanges function

This complements the clang_getSkippedRanges function which returns skipped ranges filtered by a specific file.

This function is useful when all the ranges are desired (and a lot more efficient than the equivalent of asking for the ranges file by file, since the implementation of clang_getSkippedRanges iterates over all ranges anyway).

Differential Revision: https://reviews.llvm.org/D20132

llvm-svn: 279076