[libcxx] Optimize vectors construction of trivial types from an iterator range with const-ness mismatch.

We already have a specialization that will use memcpy for construction

of trivial types from an iterator range like

std::vector<int>(int *, int *);

But if we have const-ness mismatch like

std::vector<int>(const int *, const int *);

we would use a slow path that copies each element individually. This change

enables the optimal specialization for const-ness mismatch. Fixes PR37574.

Contributions to the patch are made by Arthur O'Dwyer, Louis Dionne.

rdar://problem/40485845

Reviewers: mclow.lists, EricWF, ldionne, scanon

Reviewed By: ldionne

Subscribers: christof, ldionne, howard.hinnant, cfe-commits

Differential Revision: https://reviews.llvm.org/D48342