cmd/compile: optimize types.sconv

Now that symfmt is simpler, we can simply manually inline it into
sconv. Importantly, this allows us to avoid allocating a buffer +
writing a string + re-interning it when we don't need to qualify the
identifier.

Passes toolstash -cmp.

Updates #47087.

Change-Id: I47b57aef22301ba242556a645346f478f0c1a7d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/333162


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>