Implement the preference for move-construction over copy-construction

Implement the preference for move-construction over copy-construction
when returning an NRVO candidate expression. For example, this
properly picks the move constructor when dealing with code such as

  MoveOnlyType f() { MoveOnlyType mot; return mot; }

The previously-XFAIL'd rvalue-references test case now works, and has
been moved into the appropriate paragraph-specific test case.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@123992 91177308-0d34-0410-b5e6-96231b3b80d8