-gmodules: Don't emit incomplete breadcrumbs pointing to nonexistant PCM files.

When a module name is specified as -fmodule-name, that module gets a
clang::Module object, but it won't actually be built or imported; it
will be textual. CGDebugInfo wouldn't detect this and them emit a
DICompileUnit that had a hash but no name and that confused both
dsymutil, LLDB, and myself.

rdar://problem/47926508

Differential Revision: https://reviews.llvm.org/D57976

llvm-svn: 353578