libstdc++: Express std::vector's size() <= capacity() invariant in code

This adds optimizer hints so that GCC knows that size() <= capacity() is
always true. This allows the compiler to optimize away re-allocating
paths when assigning new values to the vector without resizing it, e.g.,
vec.assign(vec.size(), new_val).

libstdc++-v3/ChangeLog:

	* include/bits/stl_vector.h (_Vector_base::_M_invariant()): New
	function.
	(vector::size(), vector::capacity()): Call _M_invariant().
	* testsuite/23_containers/vector/capacity/invariant.cc: New test.
	* testsuite/23_containers/vector/types/1.cc: Add suppression for
	false positive warning (PR110060).