Recognize when the named return value optimization applies in a

Recognize when the named return value optimization applies in a
"return" statement and mark the corresponding CXXConstructExpr as
elidable. Teach CodeGen that eliding a temporary is different from
eliding an object construction.

This is just a baby step toward NRVO.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@103849 91177308-0d34-0410-b5e6-96231b3b80d8