Make some parallel mode algorithms usable in constexpr contexts
This makes the __parallel::equal and __parallel:lexicographical_compare
algorithms usable in constant expressions, by dispatching to the
sequential algorithm when calling during constant evaluation.
* include/parallel/algobase.h (equal, lexicographical_compare): Add
_GLIBCXX20_CONSTEXPR and dispatch to sequential algorithm when being
constant evaluated.
* include/parallel/algorithmfwd.h (equal, lexicographical_compare):
Add _GLIBCXX20_CONSTEXPR.
From-SVN: r276431