Consolidate and unify initializer list deduction

Summary:
This patch reduces duplication in the template argument deduction code
for handling deduction from initializer lists in a function call. This
extends the fix for PR12119 to also apply to the case where the
corresponding parameter is a trailing parameter pack.

Test Plan:
A test for deduction from nested initializer lists where the
corresponding parameter is a trailing parameter pack is added in
`clang/test/SemaCXX/cxx0x-initializer-stdinitializerlist.cpp`.

Reviewers: fraggamuffin, rsmith

Reviewed By: rsmith

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D10681

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@240612 91177308-0d34-0410-b5e6-96231b3b80d8