Implementation partitions bring two extra cases where we have
visibility of module-private data.
1) When we import a module implementation partition.
2) When a partition implementation imports the primary module intertace.
We maintain a record of direct imports into the current module since
partition decls from direct imports (but not trasitive ones) are visible.
The rules on decl-reachability are much more relaxed (with the standard
giving permission for an implementation to load dependent modules and for
the decls there to be reachable, but not visible).
Differential Revision: https://reviews.llvm.org/D118599
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
Iain Sandoe