[llvm] Turn off uninitialized warnings for GCC up to 14. (PR #147968)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> The current limit of GCC 12 comes from #76251, so @cjacek, @MaskRay, are you okay with raising the limit, or is it important that we keep `-Wuninitialized` on those platforms that are not affected by that GCC bug?

Clang does better job at sometimes-ininitialized warnings, and we do not want to uglify code to appease GCC false positives. I think it's reasonable to bump the version.

https://github.com/llvm/llvm-project/pull/147968