When making modules transitively visible, don't take into account

whether they have missing header files.

Whether a module's headers happen to be present on the local file system
should make no difference to whether we make its contents visible when
importing another module that re-exports it. If we have an up-to-date
AST file that we can load, that's all that matters.

This fixes the ability to header syntax checking for modular headers in
C++20 mode (or in prior modes where -fmodules-local-submodule-visibility
is enabled but -fmodules is not).