Add a module cache path to test/Headers/cxx11

Don't depend on the serialization format remaining the same.
Verify all the module map files for a pcm are the same on load

We already verified the primary module map file (either the one that
defines the top-level module, or the one that allows inferring it if it
is an inferred framework module). Now we also verify any other module
map files that define submodules, such as when there is a
module.private.modulemap file.