[Diagnostics] Resolve const-ness problems introduced by TypedNode

Since `TypedNode` elements are all marked as `const` diagnostics
need to get some of the APIs adjusted to support passing `const Expr *`.

Author: xedin

include/swift/AST/Expr.h

@@ -3759,7 +3759,12 @@ class ClosureExpr : public AbstractClosureExpr {
     assert(hasExplicitResultType() && "No explicit result type");
     return ExplicitResultType;
   }
-  
+
+  TypeRepr *getExplicitResultTypeRepr() const {
+    assert(hasExplicitResultType() && "No explicit result type");
+    return ExplicitResultType.getTypeRepr();
+  }
+
   void setExplicitResultType(SourceLoc arrowLoc, TypeLoc resultType) {
     ArrowLoc = arrowLoc;
     ExplicitResultType = resultType;