go/types, types2: quote user-supplied names in error messages

Use `' quotes (as in `foo') to differentiate from Go quotes.
Quoting prevents confusion when user-supplied names alter
the meaning of the error message.

For instance, report

        duplicate method `wanted'

rather than

        duplicate method wanted

Exceptions:
- don't quote _:
        `_' is ugly and not necessary
- don't quote after a ":":
        undefined name: foo
- don't quote if the name is used correctly in a statement:
        goto L jumps over variable declaration

Quoting is done with a helper function and can be centrally adjusted
and fine-tuned as needed.

Adjusted some test cases to explicitly include the quoted names.

Fixes #65790.

Change-Id: Icce667215f303ab8685d3e5cb00d540a2fd372ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/571396
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com>

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