I noticed that these tests had empty `main` functions. Dropping them and
renaming the tests to `MEOW.compile.pass.cpp` will slightly improve test
throughput.
Three unrelated, small improvements:
* `test_macros.h` was incorrectly saying `__has_include("<version>")`
instead of `__has_include(<version>)`.
+ This caused `<ciso646>` to always be included (noticed because MSVC's
STL emitted a deprecation warning).
+ I searched all of LLVM and found no other occurrences.
* `thread.condition.condvarany/wait_for_pred.pass.cpp` forgot to test
anything.
+ I followed what `wait_for.pass.cpp` is testing.
* Uncomment spaceship test coverage.
All supported compilers that support C++20 now support concepts. So, remove
`_LIB_LIBCPP_HAS_NO_CONCEPTS` in favor of `_LIBCPP_STD_VER > 17`. Similarly in
the tests, remove `// UNSUPPORTED: libcpp-no-concepts`.
Differential Revision: https://reviews.llvm.org/D121528
Implements parts of:
- P0898R3 Standard Library Concepts
- P1754 Rename concepts to standard_case for C++20, while we still can
Reviewed By: Mordante
Differential Revision: https://reviews.llvm.org/D98983
