libstdc++: Implement LWG 3706 for COW strings

libstdc++: Implement LWG 3706 for COW strings

The basic_string deduction guides are defined for the old ABI, but the
tests are currently disabled. This is because a single case fails when
using the old ABI, which is just because LWG 3706 isn't implemented for
the old ABI. That can be done easily, and the tests can be enabled.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h [!_GLIBCXX_USE_CXX11_ABI]
	(basic_string(const _CharT*, const _Alloc&)): Constrain to
	require an allocator-like type to fix CTAD ambiguity (LWG 3706).
	* testsuite/21_strings/basic_string/cons/char/deduction.cc:
	Remove dg-skip-if.
	* testsuite/21_strings/basic_string/cons/wchar_t/deduction.cc:
	Likewise.