libstdc++: Use RAII in std::vector::_M_default_append

Similar to r14-2052-gdd2eb972a5b063, replace the try-block with RAII
types for deallocating storage and destroying elements.

libstdc++-v3/ChangeLog:

	* include/bits/vector.tcc (_M_default_append): Replace try-block
	with RAII types.