Promote the static getNRVOCandidate() function, which computed the

NRVO candidate for a return statement, to
Sema::getCopyElisionCandidate(), and teach it enough to also determine
the NRVO candidate for a throw expression. We still don't use the
latter information, however.

Along the way, implement core issue 1148, which eliminates copy
elision from catch parameters and clarifies that copy elision cannot
occur from function parameters (which we already implemented).

llvm-svn: 123982