cmd/compile/internal/importer, gcimporter: use *TypeParam as tparamIndex map value

This is a map from identifiers to type parameters, use *TypeParam
as map value instead of Type.

Change-Id: Ib006393418c6352bcffc1c6796c5e002c33d9f4e
Reviewed-on: https://go-review.googlesource.com/c/go/+/379634


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Dan Scales <danscales@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>