cmd/compile: report an error URL with error messages

In the type checkers, add Config.ErrorURL (or Config._ErrorURL for
go/types) to configure whether and how an error message should report
a URL for errors that have an error code.

In the compiler, configure types2 to report an error URL of the form
" [go.dev/e/XXX]", where XXX stands for the error code, with the URL
appended to the first line of an error.

Rename the compiler flag -url to -errorurl. At the moment this flag
is disabled by default.

Example for a one-line error message:

<pos>: undefined: f [go.dev/e/UndeclaredName]

Example for a multi-line error message:

<pos>: not enough arguments in call to min [go.dev/e/WrongArgCount]
	have ()
	want (P, P)

Change-Id: I26651ce2c92ad32fddd641f003db37fe12fdb1cd
Reviewed-on: https://go-review.googlesource.com/c/go/+/497715
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>

0 files changed, 0 insertions, 0 deletions