encoding/xml: fix 'unsupported type' error on interface{} attributes

[silkeh]

When given interface value that is actually a type implementing
encoding.TextMarshaler or xml.MarshalerAttr, marshalAttr would return an
unsupported type error. The cause of this is that pointer and interface values
are dereferences after checking if the supported interfaces are implemented.

Solve this by moving the dereference of the pointer and interface values to the
start of the function, and update the test cases to test for this situation.

Fixes #48624

[gopherbot]

This PR (HEAD: fe20d92) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/go/+/352229 to see it.

Tip: You can toggle comments from me using the comments slash command (e.g. /comments off)
See the Wiki page for more info

[gopherbot]

Message from Go Bot:

Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
A maintainer will review your change and provide feedback. See
https://golang.org/doc/contribute.html#review for more info and tips to get your
patch through code review.

Most changes in the Go project go through a few rounds of revision. This can be
surprising to people new to the project. The careful, iterative review process
is our way of helping mentor contributors and ensuring that their contributions
have a lasting impact.

During May-July and Nov-Jan the Go project is in a code freeze, during which
little code gets reviewed or merged. If a reviewer responds with a comment like
R=go1.11 or adds a tag like "wait-release", it means that this CL will be
reviewed as part of the next development cycle. See https://golang.org/s/release
for more details.

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/352229.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

This PR (HEAD: 16f22f7) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/go/+/352229 to see it.

Tip: You can toggle comments from me using the comments slash command (e.g. /comments off)
See the Wiki page for more info