[dev.typeparams] Separate out gcshape types that are instantiated types

Distinguish the gcshape of all top-level instantiated type from normal
concrete types, even if they have the exact same underlying "shape",
because in a function instantiation, any method call on this type arg
will be a generic method call (requiring a dictionary), rather than a
direct method call on the underlying type (no dictionary).

So, we add the instshape prefix to the gcshape name for instantiated
types, and we make it a defined type with that name.

Change-Id: I33056269d24f3451a2632a5ce6a481108f533c9c
Reviewed-on: https://go-review.googlesource.com/c/go/+/335169


Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>