[clang] [clang][x86] Fix the return type of the cvtpd2dq builtin (PR #189254)

[Jean-Michel Gorius via cfe-commits]

jmgorius wrote:

Thanks @RKSimon !

Would it make sense to open more PRs if I come across inconsistent types in the x86 builtins? I found a few that, e.g., should semantically operate on unsigned values but take vectors of signed values instead (`packuswb128` comes to mind).

https://github.com/llvm/llvm-project/pull/189254