cmd/compile/internal/types2: simplify error reporting API (cleanup)

- Remove specialized errorf functions for invalid AST/argument/operation.
  Instead use prefix constants with the error message.

- Replace several calls to Checker.errorf with calls to Checker.error
  if there are no arguments to format.

- Replace a handful of %s format verbs with %v to satisfy vet check.

- Add a basic test that checks that we're not using Checker.errorf when
  we should be using Checker.error.

Change-Id: I7bc7c14f3cf774689ec8cd5782ea31b6e30dbcd6
Reviewed-on: https://go-review.googlesource.com/c/go/+/300995


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>