[clang] [clang][x86] Fix `_mm512_popcnt_epi64` constexpr test (PR #152478)

[Simon Pilgrim via cfe-commits]

RKSimon wrote:

just realised its not being run as c++ - just plain c :( Please can you add additional RUN lines?

https://github.com/llvm/llvm-project/pull/152478