[clang] Make sure argument expansion locations are correct in presence of predefined buffer

Summary:
Macro argument expansion logic relies on skipping file IDs that created
as a result of an include. Unfortunately it fails to do that for
predefined buffer since it doesn't have a valid insertion location.

As a result of that any file ID created for an include inside the
predefined buffers breaks the traversal logic in
SourceManager::computeMacroArgsCache.

To fix this issue we first record number of created FIDs for predefined
buffer, and then skip them explicitly in source manager.

Another solution would be to just give predefined buffers a valid source
location, but it is unclear where that should be..

Reviewers: sammccall

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78649