[clang] [CIR] Disable gcc partially overloaded virtual warning (PR #130322)

[Andy Kaylor via cfe-commits]

andykaylor wrote:

> Someone better equipped to review cmake should review this, but looks ok to me.

I'm not sure who is well-versed in the dark art of cmake. I made this same change in the incubator, and it's working there, so I'm confident that it works as intended.

https://github.com/llvm/llvm-project/pull/130322