Implement a simple form of the C++ named return value optimization for

Implement a simple form of the C++ named return value optimization for
return statements. We perform NRVO only when all of the return
statements in the function return the same variable. Fixes some link
failures in Boost.Interprocess (which is relying on NRVO), and
probably improves performance for some C++ applications.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@103867 91177308-0d34-0410-b5e6-96231b3b80d8