Fix tests that fail in C++2a mode

Fix tests that fail in C++2a mode

The GNU extension that allows using the wrong allocator type with a
container is disabled for C++2a mode, because the standard now requires
a diagnostic. Fix the tests that fail when -std=gnu++2a is used.

Also remove some reundant tests that are duplicates of another test
except for a target specifier of c++11. Those tests previously set
-std=gnu++11 explicitly but that was replaced globally with a target
specifier. These tests existed to verify that explicit instantiation
worked for both C++98 and C++11 modes, but now do nothing because both
copies of the test use -std=gnu++14 by default. Instead of duplicating
the test we should be regularly running the whole testsuite with
different -std options.

	* testsuite/23_containers/deque/requirements/explicit_instantiation/
	1_c++0x.cc: Remove redundant test.
	* testsuite/23_containers/deque/requirements/explicit_instantiation/
	2.cc: Use target selector instead of preprocessor condition.
	* testsuite/23_containers/deque/requirements/explicit_instantiation/
	3.cc: Do not run test for C++2a.
	* testsuite/23_containers/forward_list/requirements/
	explicit_instantiation/3.cc: Likewise.
	* testsuite/23_containers/forward_list/requirements/
	explicit_instantiation/5.cc: Do not test allocator rebinding extension
	for C++2a.
	* testsuite/23_containers/list/requirements/explicit_instantiation/
	1_c++0x.cc: Remove redundant test.
	* testsuite/23_containers/list/requirements/explicit_instantiation/
	2.cc: Use target selector instead of preprocessor condition.
	* testsuite/23_containers/list/requirements/explicit_instantiation/
	3.cc: Do not run test for C++2a.
	* testsuite/23_containers/list/requirements/explicit_instantiation/
	5.cc: Do not test allocator rebinding extension for C++2a.
	* testsuite/23_containers/map/requirements/explicit_instantiation/
	1_c++0x.cc: Remove redundant test.
	* testsuite/23_containers/map/requirements/explicit_instantiation/
	2.cc: Adjust comment.
	* testsuite/23_containers/map/requirements/explicit_instantiation/
	3.cc: Do not run test for C++2a.
	* testsuite/23_containers/map/requirements/explicit_instantiation/
	5.cc: Do not test allocator rebinding extension for C++2a.
	* testsuite/23_containers/multimap/requirements/explicit_instantiation/
	1_c++0x.cc: Remove redundant test.
	* testsuite/23_containers/multimap/requirements/explicit_instantiation/
	3.cc: Do not run test for C++2a.
	* testsuite/23_containers/multimap/requirements/explicit_instantiation/
	5.cc: Do not test allocator rebinding extension for C++2a.
	* testsuite/23_containers/multiset/requirements/explicit_instantiation/
	3.cc: Do not run test for C++2a.
	* testsuite/23_containers/multiset/requirements/explicit_instantiation/
	5.cc: Do not test allocator rebinding extension for C++2a.
	* testsuite/23_containers/set/requirements/explicit_instantiation/3.cc:
	Do not run test for C++2a.
	* testsuite/23_containers/set/requirements/explicit_instantiation/
	1_c++0x.cc: Remove redundant test.
	* testsuite/23_containers/set/requirements/explicit_instantiation/5.cc:
	Do not test allocator rebinding extension for C++2a.
	* testsuite/23_containers/unordered_map/requirements/
	explicit_instantiation/3.cc: Likewise.
	* testsuite/23_containers/unordered_map/requirements/
	explicit_instantiation/5.cc: Do not test allocator rebinding extension
	for C++2a.
	* testsuite/23_containers/unordered_multimap/requirements/
	explicit_instantiation/3.cc: Do not run test for C++2a.
	* testsuite/23_containers/unordered_multimap/requirements/
	explicit_instantiation/5.cc: Do not test allocator rebinding extension
	for C++2a.
	* testsuite/23_containers/unordered_multiset/requirements/
	explicit_instantiation/3.cc: Do not run test for C++2a.
	* testsuite/23_containers/unordered_multiset/requirements/
	explicit_instantiation/5.cc: Do not test allocator rebinding extension
	for C++2a.
	* testsuite/23_containers/unordered_set/requirements/
	explicit_instantiation/3.cc: Do not run test for C++2a.
	* testsuite/23_containers/unordered_set/requirements/
	explicit_instantiation/5.cc: Do not test allocator rebinding extension
	for C++2a.
	* testsuite/23_containers/vector/ext_pointer/explicit_instantiation/
	2.cc: Remove redundant test.
	* testsuite/23_containers/vector/ext_pointer/explicit_instantiation/
	3.cc: Do not run test for C++2a.
	* testsuite/23_containers/vector/requirements/explicit_instantiation/
	3.cc: Likewise.

From-SVN: r272001