[ADT] Clarify llvm::bit_cast implementation comment

[jloser]

Summary

When reviewing https://reviews.llvm.org/D132330, I noticed a few pre-existing
comments regarding the implementation of llvm::bit_cast. These comments are 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.

[jloser]

Only mention the C++17 vs C++20 difference - leave the trivially constructible comment alone.

[kazu]

Thanks!

[MaskRay]

LGTM.

llvm/include/llvm/ADT/bit.h

 #ifndef LLVM_ADT_BIT_H
 #define LLVM_ADT_BIT_H
 
 #include <cstring>
 #include <type_traits>
 
 namespace llvm {
 
-// This implementation of bit_cast is different from the C++17 one in two ways:
+// This implementation of bit_cast is different from the C++20 one in two ways:
 //  - It isn't constexpr because that requires compiler support.
 //  - It requires trivially-constructible To, to avoid UB in the implementation.
 template <
     typename To, typename From,
     typename = std::enable_if_t<sizeof(To) == sizeof(From)>,
     typename = std::enable_if_t<std::is_trivially_constructible<To>::value>,
     typename = std::enable_if_t<std::is_trivially_copyable<To>::value>,
     typename = std::enable_if_t<std::is_trivially_copyable<From>::value>>