[AlwaysInliner] Enable call site inlining to make flatten attribute working again (#53360)

Problem: Migration to new PM broke flatten attribute.

This is one use case why LLVM should support inlining call-site with alwaysinline.  The flatten attribute is nowdays broken, so we should either land patch like this one or remove everything related to  flatten attribute from Clang.

Second use case is something like "per call site inlining intrinsics" to control inlining even more; mentioned in
https://lists.llvm.org/pipermail/cfe-dev/2018-September/059232.html

Fixes https://github.com/llvm/llvm-project/issues/53360

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D117965