Since the serialization code would recognize modules by names and the
name of all global module fragment is <global>, so that the
serialization code would complain for the same module.
This patch fixes this by using a unique global module fragment in Sema.
Before this patch, the compiler would fail on an assertion complaining
the duplicated modules.
Reviewed By: urnathan, rsmith
Differential Revision: https://reviews.llvm.org/D115610