clang/Modules: Error if ReadASTBlock does not find the main module

If ReadASTBlock does not find its top-level submodule, there's something
wrong the with the PCM.  Error in that case, to avoid hitting problems
further from the source.

Note that the Swift compiler sometimes hits a case in
CompilerInstance::loadModule where the top-level submodule mysteriously
does not have Module::IsFromModuleFile set.  That will emit a confusing
warn_missing_submodule, which was never intended for the main module.
The recent audit of error-handling in ReadAST may have rooted out the
real problem.  If not, this commit will help to clarify the real
problem, and replace a confusing warning with an error pointing at the
malformed PCM file.

We're specifically sniffing out whether the top-level submodule was
found/processed, in case there is a malformed module file that is
missing it.  If there is an error encountered during ReadSubmoduleBlock
the return status should already propagate through.  It would be nice to
detect other missing submodules around here to catch other instances of
warn_missing_submodule closer to the source, but that's left as a future
exercise.

https://reviews.llvm.org/D70063