[libc-commits] [libc] [libc] Use `${libc_opt_high_flag}` instead of `-O3` (PR #123233)

[Petr Hosek via libc-commits]

petrhosek wrote:

> Do we want to override debug builds?

We talked about it in he libc meeting today and agreed that we shouldn't be overriding the flag and instead rely on whatever user sets in CMake (except for cases where `-O3` is absolutely necessary). I plan on doing that in a follow up PR, this change is just an initial cleanup.

https://github.com/llvm/llvm-project/pull/123233