[libclang] Introduce clang_Location_isInSystemHeader to check if a location resides in a system header.

This is a modified patch provided from Mikołaj Siedlarek!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179384 91177308-0d34-0410-b5e6-96231b3b80d8