[llvm] [CodeGen] Get rid of incorrect `std` template specializations (PR #160804)

[A. Jiang via llvm-commits]

frederick-vs-ja wrote:

@kparzysz Thanks. The requirements was in [namespace.std]/2 in C++17. I've updated the PR message and comments to make the reason of changes clearer.

https://github.com/llvm/llvm-project/pull/160804