This implements the parsing and recognition of module partition CMIs
and removes the FIXMEs in the parser.
Module partitions are recognised in the base computation of visibility,
however additional amendments to visibility follow in subsequent patches.
Differential Revision: https://reviews.llvm.org/D118586
and the global and private module fragment.
For now, the private module fragment introducer is ignored, but use of
the global module fragment introducer should be properly enforced.
llvm-svn: 358353