libstdc++: Fix some Parallel Mode testsuite failures

Some of these are due to no longer using #pragma GCC system_header in
libstdc++ headers, some have been failing for longer and weren't
noticed.

libstdc++-v3/ChangeLog:

	* include/parallel/algobase.h (search): Use sequential algorithm
	for constant evaluation.
	* include/parallel/algorithmfwd.h (search): Add
	_GLIBCXX20_CONSTEXPR.
	* include/parallel/multiway_merge.h: Remove stray semi-colon.
	* include/parallel/multiseq_selection.h: Add diagnostic pragmas
	for -Wlong-long warning.
	* include/parallel/quicksort.h: Likewise.
	* include/parallel/random_number.h: Likewise.
	* include/parallel/settings.h: Likewise.
	* include/parallel/workstealing.h: Replace ++ and -- on volatile
	variables.
	* testsuite/17_intro/names.cc: Skip names defined by
	<tr1/random>.
	* testsuite/20_util/pair/dangling_ref.cc: Skip test if Parallel
	Mode is enabled.
	* testsuite/20_util/tuple/dangling_ref.cc: Likewise.