cmd/compile: prevent importReader reading type parameter twice

The importReader always reads type parameter before declaring type stub
declaration. Thus, for recursive type, the type parameter is going to be
read twice, cause the bound more than once error.

To fix this, only read the type parameter after declaring stub obj, thus
r.doDecl can see the type was already inserted and terminate the
recursive call earlier.

Fixes #48280

Change-Id: I272e2f214f739fb8ec71a8628ba297477e1b7755
Reviewed-on: https://go-review.googlesource.com/c/go/+/349009


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>