[ClangImporter] Switch `lookupGlobalsAsMembers` to take a non-optional EffectiveClangContext
The only caller is `loadNamedMembers`, and that
passes in a non-optional EffectiveClangContext,
meaning that we'd miss the case when
`getEffectiveClangContext` returns `nullptr`, crashing
in `translateContext`. No test case unfortunately
as I haven't been able to come up with a reproducer.
rdar://129619711