libstdc++: Expand compile-time ranges tests for vector and basic_string.

libstdc++: Expand compile-time ranges tests for vector and basic_string.

This replaces most test_constexpr invocations with direct calls to
test_ranges(), which is also used for runtime tests.

SimpleAllocator was made constexpr to simplify this refactoring. Other
test allocators, like uneq_allocator (used in from_range constructor
tests), were not updated.

libstdc++-v3/ChangeLog:

	* testsuite/21_strings/basic_string/cons/from_range.cc: Replace
	test_constexpr with test_ranges inside static_assert.
	* testsuite/21_strings/basic_string/modifiers/append/append_range.cc:
	Likewise.
	* testsuite/21_strings/basic_string/modifiers/assign/assign_range.cc:
	Likewise.
	* testsuite/21_strings/basic_string/modifiers/insert/insert_range.cc:
	Likewise.
	* testsuite/21_strings/basic_string/modifiers/replace/replace_with_range.cc:
	Likewise.
	* testsuite/23_containers/vector/bool/cons/from_range.cc: Likewise.
	* testsuite/23_containers/vector/bool/modifiers/assign/assign_range.cc:
	Likewise.
	* testsuite/23_containers/vector/bool/modifiers/insert/insert_range.cc:
	Likewise.
	* testsuite/23_containers/vector/cons/from_range.cc: Likewise.
	* testsuite/23_containers/vector/modifiers/assign/assign_range.cc:
	Likewise.
	* testsuite/23_containers/vector/modifiers/insert/insert_range.cc:
	Likewise.
	* testsuite/23_containers/vector/bool/modifiers/insert/append_range.cc:
	Run full test_ranges instead of span-only in test_constexpr.
	* testsuite/23_containers/vector/modifiers/append_range.cc:
	Replace test_constexpr with calls to test_ranges and test_overlapping.
	* testsuite/util/testsuite_allocator.h (__gnu_test::SimpleAllocator):
	Declared member functions as constexpr.