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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

I haven't analyzed the thing that it warns about in detail, but the warnings are very very noisy and disruptive.

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

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