[libc++] Unify __is_trivial_equality_predicate and __is_trivial_plus_operation...

[libc++] Unify __is_trivial_equality_predicate and __is_trivial_plus_operation into __desugars_to (#68642)

When working on an OpenMP offloading backend for standard parallel
algorithms (https://github.com/llvm/llvm-project/pull/66968

) we noticed
the need of a generalization of `__is_trivial_plus_operation`. This patch
merges `__is_trivial_equality_predicate` and `__is_trivial_plus_operation`
into `__desugars_to`, and in the future we might extend the latter to support
other binary operations as well.

Co-authored-by: Louis Dionne <ldionne.2@gmail.com>