STL: Test /std:c++latest /permissive (lax) mode

[StephanTLavavej]

The compiler recently implemented an explicit /permissive (lax) option.

An expected upcoming compiler change (Microsoft-internal MSVC-PR-221404) will make /std:c++latest imply /permissive- (strict). However, /std:c++latest /permissive (lax) will be a supported combination, and will be required by some of our users.

• We'll need to change our test coverage accordingly. (Currently, we test /std:c++latest by itself, and /std:c++latest with /permissive- (strict), but not /std:c++latest /permissive (lax) because that option previously didn't exist.) We can update our tests immediately, to immunize them against the upcoming change to /std:c++latest.

• We'll also need to figure out what to do about portions of the STL (like ranges) that absolutely cannot tolerate /permissive (lax) mode, since ranges is used by other components like span, and /permissive (lax) versus /permissive- (strict) cannot currently be detected at preprocessor time.

[CaseyCarter]

A compiler dev asked me yesterday for help fixing the std::variant test that is broken by this change, strongly suggesting this will happen soon™. I'm preparing a fix that will make every /std:c++latest test explicitly /permissive or /permissive-.