[libc++] ADL-proof <vector> by adding _VSTD:: qualification on calls.

(1) Add _VSTD:: qualification to __swap_allocator.

(2) Add _VSTD:: qualification consistently to __to_address.

(3) Add some more missing _VSTD:: to <vector>, with a regression test.
This part is cleanup after d9a4f936.

Note that a vector whose allocator actually runs afoul of any of these ADL calls will
likely also run afoul of simple things like `v1 == v2` (which is also an ADL call).
But, still, libc++ should be consistent in qualifying function calls wherever possible.

Relevant blog post: https://quuxplusone.github.io/blog/2019/09/26/uglification-doesnt-stop-adl/

Differential Revision: https://reviews.llvm.org/D91708