[NFC] Add <initializer_list> to the synopsis of <utility>

Summary:
It is part of the synopsis in the Standard and <utility> does include it,
but it was left out of the synopsis comment.

Reviewers: EricWF, mclow.lists

Subscribers: christof, llvm-commits

Differential Revision: https://reviews.llvm.org/D48611

llvm-svn: 336368