AST: Mangle reference temporaries reliably

Summary:
Previously, we would generate a single name for all reference
temporaries and allow LLVM to rename them for us.  Instead, number the
reference temporaries as we build them in Sema.

Reviewers: rsmith

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D3554

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207776 91177308-0d34-0410-b5e6-96231b3b80d8