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