[llvm] [CMake] Disable more noisy GCC warnings (PR #159044)

[Oliver Stöneberg via llvm-commits]

firewave wrote:

> With this in place, building LLVM with GCC 13 "only" produces 47 warnings, but those are less spammy and seemingly accurate - and potentially actionable.

Seems reasonable to disable them for now (although I have no idea what the policy might be on this) so you have manageable output from the compiler.

https://github.com/llvm/llvm-project/pull/159044