cmd/compile: pass type checker error codes in the compiler

Pass type checker error codes to base.ErrorfAt function calls
in the compiler (but don't do anything yet with the code).

Also, provide error codes to base.ErrorfAt calls in the
compiler as needed.

This opens the door towards reporting the error code and/or
providing a link/reference to more detailed explanations
(see internal/types/errors/codes.go).

Change-Id: I0ff9368d8163499ffdac6adfe8331fdc4a19b4b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/475198


Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>