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

[Matt Arsenault via cfe-commits]

arsenm wrote:

> 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.

The compiler doesn't synthesize many loops. The only case I can think of is the inline expansion path for memcpy/memmove/memset?

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