Allow builtins as yul identifier paths in antlr grammar.

test/libsolidity/syntaxTests/inlineAssembly/assignment_to_function_pointer.sol

@@ -0,0 +1,9 @@
+contract C {
+  function f() public pure {
+    function() external g;
+    assembly {
+      g.address := 0x42
+      g.selector := 0x23
+    }
+  }
+}