Use the correct from type in a SCS

[• ehsan]

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. It doesn't completely fix that
test case though.

[• ehsan]

Closed by commit rL213680 (authored by @ehsan).

cfe/trunk/lib/Sema/SemaOverload.cpp

static bool IsStandardConversion(Sema &S, Expr* From, QualType ToType,
   if (FromType == S.Context.OverloadTy) {
     DeclAccessPair AccessPair;
     if (FunctionDecl *Fn
           = S.ResolveAddressOfOverloadedFunction(From, ToType, false,
                                                  AccessPair)) {
       // We were able to resolve the address of the overloaded function,
       // so we can convert to the type of that function.
       FromType = Fn->getType();
+      SCS.setFromType(FromType);
 
       // we can sometimes resolve &foo<int> regardless of ToType, so check
       // if the type matches (identity) or we are converting to bool
       if (!S.Context.hasSameUnqualifiedType(
                       S.ExtractUnqualifiedFunctionType(ToType), FromType)) {
         QualType resultTy;
         // if the function type matches except for [[noreturn]], it's ok
         if (!S.IsNoReturnConversion(FromType,