[clang] [Headers][X86] Allow pmuludq/pmuldq to be used in constexpr (PR #153293)

[Simon Pilgrim via cfe-commits]

RKSimon wrote:

> Should I wait for @phoebewang's review before merging?

If you want a second opinion its fine to wait, but you're under no obligation to wait.

https://github.com/llvm/llvm-project/pull/153293