[libclang] Allow skipping warnings from all included files

Depending on the included files and the used warning flags, e.g. -
Weverything, a huge number of warnings can be reported for included
files. As processing that many diagnostics comes with a performance
impact and not all clients are interested in those diagnostics, add a
flag to skip them.

Differential Revision: https://reviews.llvm.org/D48116

llvm-svn: 363067