[modules] When checking the include guard for a header, check whether it's

visible in the module we're considering entering. Previously we assumed that if
we knew the include guard for a modular header, we'd already parsed it, but
that need not be the case if a header is present in the current module and one
of its dependencies; the result of getting this wrong was that the current
module's submodule for the header would end up empty.

llvm-svn: 241953