These are identified by misc-include-cleaner.  I've filtered out those
that break builds.  Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.

* Have clang always append & pass System/Library/SubFrameworks when determining default sdk search paths.
* Teach clang-installapi to traverse there for framework input.
* Teach llvm-readtapi that the library files (TBD or binary) in there should be considered private.

resolves: rdar://137457006

Match TAPI behavior and allow input headers to be resolved via a passed
directory, which is expected to be a library sitting in a build
directory.