[Modules] Find PrivateHeaders when looking into subframeworks

Fix the current parsing of subframeworks in modulemaps to lookup for
headers based on whether they are frameworks.

rdar://problem/30563982

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@298391 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit d49e8364d3107404aeea2d4ffae3bc8cd8830366)
12 files changed