[Sema] Remove unused field from OverloadCandidate.

The only use in-tree I can find for BuiltinTypes.ResultTy is a single
store to it. We otherwise just recompute what it should be later on (and
sometimes do things like argument conversions in the process of
recomputing it).

Since it's impossible to test if the value stored there is sane, and we
don't use it anyway, we should probably just drop the field.

I'll do a follow-up patch to rename BuiltinTypes.ParamTypes ->
BuiltinParamTypes in a bit. Wanted to keep this patch relatively
minimal.

Thanks to Petr Kudryavtsev for bringing this up!

llvm-svn: 304996