[C++20][Modules][4/8] Handle generation of partition implementation CMIs.

Partition implementations are special, they generate a CMI, but it
does not have an 'export' line, and we cannot export anything from the
it [that is it can only make decls available to other members of the
owning module, not to importers of that].

Add initial testcases for partition handling, derived from the examples in
Section 10 of the C++20 standard, which identifies what should be accepted
and/or rejected.

Differential Revision: https://reviews.llvm.org/D118587