[clang] [C] Modify -Wdefault-const-init (PR #137961)

[Nathan Chancellor via cfe-commits]

https://github.com/nathanchance approved this pull request.

Thanks, this should allow us to try and keep `-Wdefault-const-init-var-unsafe` enabled while disabling `-Wdefault-const-init-field-unsafe`.

https://github.com/llvm/llvm-project/pull/137961