This adds a test that checks if the using declaration in classes still works as intended with modules.
The motivation for this is that we tried to add a shortcut to removeDecl that would skip the removal of declarations from the lookup table if they are hidden. This optimization passed the clang test suite but actually broke the using declaration in combination with -fmodules-local-submodule-visibility. In this mode we hide all decls from other modules such as by chance the parent method, in which case don't remove the parent method from the lookup table and get ambiguous lookup errors. After this patch we now correctly see if this behavior is broken by a patch like this in the test suite.