[clang] [Clang][kcfi] Sign extend KCFI typeid rather than zero extend (PR #136734)

[Bill Wendling via cfe-commits]

bwendling wrote:

> Unfortunately, the `__kcfi_typeid_` values are intentionally zero-extended, so they can be used in relocations with `movl`. With your patch applied:

Okay, so what should we do about the testcase? I'm hitting it with the 'kcfi-salt' testcases, which are loosely based on `kcfi.c`.

https://github.com/llvm/llvm-project/pull/136734