Fix initial diagnostic state setup for an explicit module with no diagnostic pragmas.

If a file has no diagnostic pragmas, we build its diagnostic state lazily, but
in this case we never set up the root state to be the diagnostic state in which
the module was originally built, so the diagnostic flags for files in the
module with no diagnostic pragmas were incorrectly based on the user of the
module rather than the diagnostic state when the module was built.

llvm-svn: 301846