libstdc++: Make move ctor noexcept for fully-dynamic string

libstdc++: Make move ctor noexcept for fully-dynamic string

The move constructor for the "fully-dynamic" COW string is not noexcept,
because it allocates a new empty string rep for the moved-from string.
However, there is no need to do that, because the moved-from string does
not have to be left empty. Instead, implement move construction for the
fully-dynamic case as a reference count increment, so the string is
shared.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* include/bits/cow_string.h [_GLIBCXX_FULLY_DYNAMIC_STRING]
	(basic_string(basic_string&&)): Add noexcept and avoid
	allocation, by sharing rep with the rvalue string.