cmd/compile/internal/typecheck: export Implements

cmd/compile/internal/typecheck: export Implements

Provide an exported version of implements to easily check if a type
implements an interface. This will be use for PGO devirtualization.

Even within the package, other callers can make use of this simpler API
to reduce duplication.

For #59959.

Change-Id: If4eb86f197ca32abc7634561e36498a247b5070f
Reviewed-on: https://go-review.googlesource.com/c/go/+/495915
Auto-Submit: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>