FunctionDefinition.resolveVirtual(): Skip unimplemented functions when lookup happens via super

test/libsolidity/syntaxTests/functionCalls/call_unimplemented_base_via_super.sol

@@ -0,0 +1,18 @@
+abstract contract I {
+    function a() internal view virtual returns(uint256);
+}
+
+abstract contract C is I {
+    function f() public view returns(uint256) {
+        return I.a();
+    }
+}
+
+abstract contract D is I {
+    function f() public view returns(uint256) {
+        return super.a();
+    }
+}
+// ----
+// TypeError 7501: (172-177): Cannot call unimplemented base function.
+// TypeError 9582: (278-285): Member "a" not found or not visible after argument-dependent lookup in type(contract super D).