Use the correct from type in a SCS

Summary:
If during constructing a standard conversion sequence, we resolve an
overload, we need to adjust the from type in the SCS according to the
resolved operator.

I found this bug when debugging PR20218.  This doesn't seem to be
observable, so there is no good way of testing it.

Reviewers: rsmith

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D4402