[dev.typeparams] go/types: move struct checking into separate file
This is a port of CL 321589 to go/types. Specifically, the same checker
methods were moved.
Change-Id: If07d96faa77d2f9409d8895f970149c42cbfe440
Reviewed-on: https://go-review.googlesource.com/c/go/+/324753
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>