[C++20][Modules][7/8] Find the primary interface name for a module.

When we are building modules, there are cases where the only way to determine
validity of access is by comparing primary interface names.  This is because we need
to be able to associate a primary interface name with an imported partition, but
before the primary interface module is complete - so that textual comparison is
necessary.

If this turns out to be needed many times, we could cache the result, but it seems
unlikely to be significant (at this time); cases with very many imported partitions
would seem unusual.

Differential Revision: https://reviews.llvm.org/D118598