[c++20] Parsing support for module-declarations, import-declarations,

[c++20] Parsing support for module-declarations, import-declarations,
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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@358353 91177308-0d34-0410-b5e6-96231b3b80d8