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

llvm-svn: 207776