[libc++] Fix PR34898 - vector iterator constructors and assign method perform...

[libc++] Fix PR34898 - vector iterator constructors and assign method perform push_back instead of emplace_back.

Summary:
The constructors `vector(Iter, Iter, Alloc = Alloc{})` and `assign(Iter, Iter)` don't correctly perform EmplaceConstruction from the result of dereferencing the iterator. This results in them performing an additional and unneeded copy.

This patch addresses the issue by correctly using `emplace_back` in C++11 and newer.

There are also some bugs in our `insert` implementation, but those will be handled separately. 

@mclow.lists We should probably merge this into 5.1, agreed?

Reviewers: mclow.lists, dlj, EricWF

Reviewed By: mclow.lists, EricWF

Subscribers: cfe-commits, mclow.lists

Differential Revision: https://reviews.llvm.org/D38757

llvm-svn: 315994