[dev.typeparams] cmd/compile/internal/types2: move interface checking into separate file
This only moves functionality from one file into another.
Except for import adjustments there are no changes to the
code.
Change-Id: Ia7d611d3a01c1ed3331dcc7cfe94a96f87b338e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/321549
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>