When building a module from a module map that isn't simply an umbrella

header, create our own in-memory buffer to parse all of the
appropriate headers, and use that to build the module. This isn't
end-to-end testable yet; that's coming next.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@144797 91177308-0d34-0410-b5e6-96231b3b80d8