Provide selector for some internal functions.

test/libsolidity/semanticTests/constants/function_unreferenced.sol

@@ -0,0 +1,11 @@
+contract B {
+    function g() public {}
+}
+contract C is B {
+    bytes4 constant s2 = B.g.selector;
+    function f() external pure returns (bytes4) { return s2; }
+}
+// ====
+// compileViaYul: also
+// ----
+// f() -> 0xe2179b8e00000000000000000000000000000000000000000000000000000000