Detect when mapping a #include/#import over to a submodule ends up

hitting a submodule that was never actually created, e.g., because
that header wasn't parsed. In such cases, complain (because the
module's umbrella headers don't cover everything) and fall back to
including the header.

Later, we'll add a warning at module-build time to catch all such
cases. However, this fallback is important to eliminate assertions in
the ASTWriter when this happens.

llvm-svn: 146933