[Clang] Updating handling of defaulted comparison operators to reflect changes from P2448R2

Prior to P2448R2 we were more aggressive in diagnosing ill-formed
constexpr functions. Many of these restrictions were relaxed and now it
is not required for defaulted comparison operators to call constexpr
functions.

This behavior is extended to before C++23 and diagnostic for it's use
can be enabled w/ -pedantic or -Wc++2b-default-comp-relaxed-constexpr

This fixes: https://github.com/llvm/llvm-project/issues/61238

Differential Revision: https://reviews.llvm.org/D146090