Add consistency query flagging unexpected frontend errors.

ql/test/consistency/UnexpectedFrontendErrors.ql

@@ -0,0 +1,16 @@
+/**
+ * @name Unexpected frontend error
+ * @description This query produces a list of all errors produced by the Go frontend
+ *              during extraction, except for those occurring in files annotated with
+ *              "// codeql test: expect frontend errors".
+ * @id go/unexpected-frontend-error
+ */
+
+import go
+
+from Error e
+where
+  not exists(Comment c | c.getFile() = e.getFile() |
+    c.getText().trim() = "codeql test: expect frontend errors"
+  )
+select e

ql/test/consistency/test.go

@@ -0,0 +1,7 @@
+package main
+
+// Example file with a syntax error to demonstrate use of "expect frontend errors" directive
+
+// codeql test: expect frontend errors
+
+This is not a valid Go program