[modules] Add an experimental -cc1 feature to embed the contents of an input

file in the .pcm files. This allows a smaller set of files to be sent to a
remote build worker when building with explicit modules (for instance, module
map files need not be sent along with the corresponding precompiled modules).

This doesn't actually make the embedded files visible to header search, so
it's not useful as a packaging format for public header files.

llvm-svn: 245028