Implement LWG-3655: The INVOKE operation and union types

[fsb4000]

Fixes #3424

[StephanTLavavej]

No change requested: this LWG issue resolution is being implemented unconditionally (as usual), but the test being extended here is C++23:

STL/tests/std/tests/P2136R3_invoke_r/env.lst

Line 4 in 86acfb1

RUNALL_INCLUDE ..\usual_latest_matrix.lst

This means that the product code is active for C++14/17/20, but is not being tested in those modes. We have tests that are built down to C++14 mode (e.g. Dev11_0535636_functional_overhaul is where INVOKE is mostly tested).

However, I don't think this coverage needs to be moved - even with my paranoia about compiler bugs, I find it hard to imagine a compiler bug that would affect this code that would appear only in C++14/17/20 mode. Also, it's slightly annoying to test anything here in C++14 mode as all that's available there is the older/deprecated result_of. So I'm simply noting this for future consideration.

[StephanTLavavej]

Thanks! I've pushed a nitpicky rename and an expansion of the test coverage. FYI @CaseyCarter.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for extending INVOKE to handle even more types! 📈 🎉 💯