[libc++] Make sure that vector copy-construction is disabled for non-copyable types

The Standard requires the value_type of the vector to be Cpp17CopyInsertable
in order for copy-construction to be enabled:

	http://eel.is/c++draft/container.requirements#tab:container.req

rdar://problem/56674564

Differential Revision: https://reviews.llvm.org/D74251