go/types, types2: distinguish between substring and regexp error patterns

go/types, types2: distinguish between substring and regexp error patterns

Use ERROR for substrings, and ERRORx for regexp error patterns.
Correctly unquote patterns for ERROR and ERRORx.
Adjust all tests in internal/types/testdata and locally as needed.

The changes to internal/types/testdata were made through
repeated applications of regexpr find/replace commands
and manual cleanups.

Fixes #51006.

Change-Id: Ib9ec5001243b688bf5aee56b7d4105fb55999ab4
Reviewed-on: https://go-review.googlesource.com/c/go/+/455755
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>