[modules] Correctly overload getModule in the MultiplexExternalSemaSource

Summary:
The MultiplexExternalSemaSource doesn't correctly overload the `getModule` function,
causing the multiplexer to not forward this call as intended.

Reviewers: v.g.vassilev

Reviewed By: v.g.vassilev

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D39416

llvm-svn: 323122