[clang] [clang] move -Wcast-function-type under -Wextra (PR #77178)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

This should have a release note and I think this is a potentially breaking change since folks using `Wextra` may get this diagnostic now.

https://github.com/llvm/llvm-project/pull/77178