[Inliner] Handle `mustprogress` functions

When inlining `mustprogress` functions, if the caller or the callee has
the attribute, we drop the function attribute. The loops that have the
`llvm.loop.mustprogress` metadata keep their metadata. We do not need to
add new loop metadata to inlined functions because the patch in D86841
already adds the relevant loop metadata in all of the necessary places.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D87262