swift234

Fix vtable thunk generation for cases with optional return values.

With the test case included in this commit we were first hitting an
assert due to a cast<FuncDecl>(decl) that failed because the supplied
decl was for a constructor, and after fixing this we failed verification
due to using the wrong result type for the generated thunk.

The result should have the same kind of 'optionalness' as the base-class
method's result, but if the type is some kind of optional it should be
wrapping the same type as the thunked-to function.

Fixes at least part of rdar://problem/20663080. I need to go back and
patch an older toolset to see if this completely resolves the issue.

Swift SVN r28289