This implements the provisions of [module.global.frag] p3/4. This provides for discarding
declarations in the global module fragment if they are not reachable from any use within
the named module purview.
Assuming that the current TU has a global module fragment, we start out by creating decls
in that fragment marked as "ModuleDiscardable".
Then when in the module purview of the named module, whenever we encounter a use
or reference of a declaration, we check to see if that declaration (or any other declaration that
becomes indirectly reachable via it) is on the GMF. If so, then we mark that declaration as
reachable. We also check for cases that types in such declarations cause type decls in the
GMF to be reachable.
In the most general case, this can represent a significant amount of work - walking through
the bodies of functions to check for indirect reachability. To counter that as far as possible
we use two sets (one for decls, one for types) to ensure that each is only visited once.