Make allocator propagation more consistent for operator+(basic_string) (P1165R1)

Make allocator propagation more consistent for operator+(basic_string) (P1165R1)

2019-05-01  Nina Dinka Ranns  <dinka.ranns@gmail.com>

	Make allocator propagation more consistent for
	operator+(basic_string) (P1165R1)
	* include/bits/basic_string.h
	(operator+(basic_string&&, basic_string&&): Changed resulting
	allocator to always be the one from the first parameter.
	* include/bits/basic_string.tcc
	(operator+(const _CharT*, const basic_string&)): Changed
	resulting allocator to be SOCCC on the second parameter's allocator.
	(operator+(_CharT, const basic_string&)): Likewise.
	* testsuite/21_strings/basic_string/allocator/char/operator_plus.cc:
	New.
	* testsuite/21_strings/basic_string/allocator/wchar_t/operator_plus.cc:
	New.

From-SVN: r270953