`<__msvc_chrono.hpp>`: `duration_cast` only needs compile time branches

[frederick-vs-ja]

The conditions of the if statements in duration_cast are all constant expressions, so we can change all of these occurences of plain if to if constexpr.

STL/stl/inc/__msvc_chrono.hpp

Lines 428 to 447 in ed8150e

	constexpr bool _Num_is_one = _CF::num == 1;
	constexpr bool _Den_is_one = _CF::den == 1;
	if (_Den_is_one) {
	if (_Num_is_one) {
	return static_cast<_To>(static_cast<_ToRep>(_Dur.count()));
	} else {
	return static_cast<_To>(
	static_cast<_ToRep>(static_cast<_CR>(_Dur.count()) * static_cast<_CR>(_CF::num)));
	}
	} else {
	if (_Num_is_one) {
	return static_cast<_To>(
	static_cast<_ToRep>(static_cast<_CR>(_Dur.count()) / static_cast<_CR>(_CF::den)));
	} else {
	return static_cast<_To>(static_cast<_ToRep>(
	static_cast<_CR>(_Dur.count()) * static_cast<_CR>(_CF::num) / static_cast<_CR>(_CF::den)));
	}
	}
	}

Originally posted by achabense in #3914 (comment)

This issue is intended for a new contributor (especially one new to GitHub) to get started with the simplest possible change.

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[achabense]

Additional background: though this is a C++17 feature, we are free to add if constexpr wherever suitable (see #189).