Fix incorrect call to ExprResult::get()

[arichardson]

Summary

Res is already a ExprResult, so if we call .get() we convert an
ExprError() result into an unset result. I discovered this in our
downstream CHERI target where this resulted in a crash due to a
NULL-dereference.
It appears that this was introduced in SVN revision 201788 (8690a6860a45ba36e39b4ff0dbe434195e125d11)

[Anastasia]

LGTM! Thanks! Great spot!

clang/lib/Sema/SemaExpr.cpp

 /// performed on a function designator of a call expression.
 ExprResult Sema::CallExprUnaryConversions(Expr *E) {
   QualType Ty = E->getType();
   ExprResult Res = E;
   // Only do implicit cast for a function type, but not for a pointer
   // to function type.
   if (Ty->isFunctionType()) {
     Res = ImpCastExprToType(E, Context.getPointerType(Ty),
-                            CK_FunctionToPointerDecay).get();
+                            CK_FunctionToPointerDecay);
     if (Res.isInvalid())
       return ExprError();
   }
   Res = DefaultLvalueConversion(Res.get());
   if (Res.isInvalid())
     return ExprError();
   return Res.get();
 }