[clang] clang: Emit llvm.loop.align metadata for -falign-loops (PR #223147)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

This isn't precisely equivalent; the backend option applies to all loops the backend can see.  Applying loop metadata only applies to loops the frontend can see.

Not sure how frequently the distinction matters.  If we care, we could add a function attribute.

https://github.com/llvm/llvm-project/pull/223147