Enable __cpp_lib_concepts for EDG, part 1

[cpplearner]

This PR makes __cpp_lib_concepts defined in C++20 mode for all supported compilers, after adding workarounds for various EDG concepts bugs. I have opened two follow-up PRs for non-essential and largely mechanical clean-ups.

• Part 2 (Enable __cpp_lib_concepts for EDG, part 2 #4297) replaces concepts_20_matrix and concepts_latest_matrix with usual_20_matrix and usual_latest_matrix respectively.
• Part 3 (Enable __cpp_lib_concepts for EDG, part 3 #4298) replaces defined(__cpp_lib_concepts) in preprocessing conditions with _HAS_CXX20, and then eliminates some of them where they are redundant.

[StephanTLavavej]

• Pushed a source-conflict-free merge with main to pick up <expected>: Make copy/move assignment operators of expected propagate triviality #4271.
• Fully reduced and reported VSO-1949451 "EDG concepts rejects std::expected trying to propagate triviality of assignment operations".
• Recorded that in tracking issue Report EDG bugs #1621.
• Disabled the affected top-level scenarios for EDG in P0323R12_expected.

They were TrivialityScenario4 "Only destruction and move construction/assignment are trivial" and TrivialityScenario6 "All operations are trivial", revealing @CaseyCarter's deep wisdom in #4271 (comment) of testing these scenarios. 😻

[StephanTLavavej]

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

[StephanTLavavej]

I've pushed additional commits:

• Attempt to avoid timeouts in P0896R4_ranges_alg_find_end.
  • Encountered in the internal test harness, probably because the "checked" compiler consumes more memory.
• Drop "assert bug so we'll notice when it's fixed".
  • I know this had worthy intentions, but it's a huge headache when either MSVC or EDG devs are trying to test their fixed compilers.
• Properly detect MSVC, part 1.
  • DevCom-10265237 "Parameter pack size mismatch doesn't make the require-expression false (regression)" is an MSVC bug.
  • This preprocessor logic is guarding an additional constraint that MSVC needs, so I'm updating the #endif comment to ^^^ workaround ^^^.
  • Previously, I believe this code was mentally saying, "we're in concepts code, so we only need to distinguish Clang from MSVC". This is no longer the case. Now we need !defined(__clang__) && !defined(__EDG__) to prevent EDG from using a workaround that it doesn't need.
• Properly detect MSVC, part 2.
  • DevCom-1691516 "std::vector::emplace_back does not work for a class with default initializer" is an MSVC bug.
  • Curiously, this preprocessor logic was checking only #ifdef __EDG__ to distinguish it from MSVC. (I think it might have been written before Clang supported construct_at()?)
  • Instead, we need to check #if defined(__clang__) || defined(__EDG__) and send them to the "no workaround" case. This prevents Clang from using a workaround that it doesn't need.

I have comment cleanups for a followup PR but I didn't want to further randomize this one when it's so close to landing.

[StephanTLavavej]

😻 🎉 🐈 🚀 🍰

Thank you @cpplearner and @CaseyCarter for the massive amount of work needed to get to this long-awaited day!