[llvm] Replace compareMemory() with memcmp(); NFC (PR #126730)

[Aaron Ballman via llvm-commits]

AaronBallman wrote:

> > We are not aware of any C standard library which takes advantage of this former UB, so this workaround is no longer needed.
> 
> While the standard library itself doesn't, GCC does take advantage of this, so I'm not sure this is entirely safe (yet).

Ah, when I looked at the `-E` output, the nonnull attribute was offscreen, so I missed it. Good point, this is premature. Closing for now.

https://github.com/llvm/llvm-project/pull/126730