For example, `std::ranges::range<Holder<Incomplete>*>` should be
well-formed false, not a hard error at compile time.
Differential Revision: https://reviews.llvm.org/D116239
Some individual test files verify the CPO under test satisfies
`semiregular` concept. This is redundant since it is already part of the test
in verifying whether the entity is indeed a CPO in
`libcxx/test/std/library/description/conventions/customization.point.object/cpo.compile.pass.cpp`.
Differential Revision: https://reviews.llvm.org/D116173
Arthur O'Dwyer