Recover better from an incompatible .pcm file being provided by -fmodule-file=.

Recover better from an incompatible .pcm file being provided by -fmodule-file=.
We try to include the headers of the module textually in this case, still
enforcing the modules semantic rules. In order to make that work, we need to
still track that we're entering and leaving the module. Also, if the module was
also marked as unavailable (perhaps because it was missing a file), we
shouldn't mark the module unavailable -- we don't need the module to be
complete if we're going to enter it textually.

llvm-svn: 288449