Fix false positives in -Wmsvc-include by continuing header search

This makes Clang and LLVM -Wmsvc-include clean.

I believe the correct behavior here is to avoid updating the cache when
we find the header via MSVC's search rules.

Differential Revision: http://llvm-reviews.chandlerc.com/D2733

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201615 91177308-0d34-0410-b5e6-96231b3b80d8