libstdc++: Fix fully-dynamic-string build

My last change to the fully-dynamic-string actually broke it. This fixes
the move constructor so it builds, and simplifies it slightly so that
more code is common between the fully-dynamic enabled/disabled cases.

libstdc++-v3/ChangeLog:

	* include/bits/cow_string.h (basic_string(basic_string&&)): Fix
	mem-initializer for _GLIBCXX_FULLY_DYNAMIC_STRING==0 case.
	* testsuite/21_strings/basic_string/cons/char/noexcept_move_construct.cc:
	Remove outdated comment.
	* testsuite/21_strings/basic_string/cons/wchar_t/noexcept_move_construct.cc:
	Likewise.