[clang][NewPM] Fixing remaining -O0 tests that are broken under new PM
- CodeGen/flatten.c will fail under new PM becausec the new PM AlwaysInliner seems to intentionally inline functions but not call sites marked with alwaysinline (D23299)
- Tests that check remarks happen to check them for the inliner which is not turned on at O0. These tests just check that remarks work, but we can make separate tests for the new PM with -O1 so we can turn on the inliner and check the remarks with minimal changes.
Differential Revision: https://reviews.llvm.org/D62225