P0620 follow-up: deducing `auto` from braced-init-list in new expr

Summary:
This is a side-effect brought in by p0620r0, which allows other placeholder types (derived from `auto` and `decltype(auto)`) to be usable in a `new` expression with a single-clause //braced-init-list// as its initializer (8.3.4 [expr.new]/2).  N3922 defined its semantics.

References:
 http://wg21.link/p0620r0
 http://wg21.link/n3922

Reviewers: rsmith, aaron.ballman

Reviewed By: rsmith

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D39451

llvm-svn: 320401