[ADT] Clarify llvm::bit_cast implementation comment

When reviewing https://reviews.llvm.org/D132330, I noticed a few pre-existing
comments regarding the implementation of `llvm::bit_cast`. One comment is a bit
misleading since `std::bit_cast` is a C++20 standard library thing, not a
C++17 one (otherwise we could use it directly). Clarify that in the comment.

Differential Revision: https://reviews.llvm.org/D132332