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.

llvm-svn: 123992