[dev.typeparams] cmd/compile: get rid of concretify use for bounds.

We just need to substitute from the typeparams to the shapes for the dst
type of the bound.

Removed concretify substituter, not used anymore. Also removed
shape2params, not needed anymore.

However, since the dst type is now not concrete, this gives more cases
where the linker can't find a method.

I realized that we need to call MarkUsedIfaceMethod to mark a method as
used on a particular interface, else a type's method can be still
deadcoded even though MarkTypeUsedInInterface has been called on the
concrete type. I added a new version MarkUsedIfaceMethodIndex to fit my
use case.

Change-Id: Id67b72b350889dd3688b42739c337d5d79a0d1a2
Reviewed-on: https://go-review.googlesource.com/c/go/+/337230


Trust: Dan Scales <danscales@google.com>
Reviewed-by: Keith Randall <khr@golang.org>