[HeaderSearch] Fix issue where if a headermap entry maps the filename to a framework import (non-absolute path)
then we fail to find it if it is re-included later on.

rdar://16285490

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203542 91177308-0d34-0410-b5e6-96231b3b80d8