Followup to D116964 where we only did this in the CGSCC inliner. Fixes leaks reported in D116964.
Add module level inliner, which is a minimum viable product at this point. Also add some tests for it. RFC: https://lists.llvm.org/pipermail/llvm-dev/2021-August/152297.html Reviewed By: kazu Differential Revision: https://reviews.llvm.org/D106448