Silence MSVC warnings when including <bit> pre-C++20

[eramongodb]

Followup to #1412.

Addresses the following annoyingly-aggressive warning emitted by STL <bit> pre-C++20 despite using the __cpp_lib_endian feature test macro correctly (CE):

The contents of <bit> are available only with C++20 or later.

Conditioning on __cplusplus >= 202002L (C++20) is not enough due to /Zc:__cplusplus issues somehow resulting in an even worse outcome (CE):

error C3083: 'endian': the symbol to the left of a '::' must be a type
error C2039: 'native': is not a member of 'std'

Therefore, _MSVC_LANG >= 202002L is used instead, but only when defined(_MSVC_LANG) is also true, since this is an MSVC-specific issue which is not applicable to other compilers (CE).