Add additional test cases.

test/libsolidity/syntaxTests/inheritance/override/function_state_variable.sol

@@ -0,0 +1,2 @@
+interface ERC20 { function x() external returns (uint); }
+contract C is ERC20 { uint public x; }

test/libsolidity/syntaxTests/inheritance/override/state_variable_function.sol

@@ -0,0 +1,8 @@
+contract A {
+	uint public x;
+}
+contract C is A {
+	function x() public returns (uint);
+}
+// ----
+// DeclarationError: (50-85): Identifier already declared.