[clang] Teardown new PM data structures before running codegen pipeline

Do this by refactoring the optimization and codegen pipelines into separate functions.

This saves a tiny bit of memory in non-LTO builds [1].

[1] https://llvm-compile-time-tracker.com/compare.php?from=fbddf22ef72d3c2e9b14e1501841b03380eef12b&to=cd276df52eb6f2b84a8e1efe5318460c6debf82d&stat=max-rss

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D111582