AST: Update reference temporary mangling

Summary:
Update our mangling to match the discussion on cxx-abi-dev.

This involves using a seq-id instead of an optional number.

Reviewers: rsmith

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D3631

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208140 91177308-0d34-0410-b5e6-96231b3b80d8