[dev.typeparams] cmd/compile: do transformCall with non-shape type of call

Do the transformCall using the original types2-derived type of the call
(in particular, the types of the params as non-shapes). Currently, since
we were using the param types of the instantiation, we might add in
interface conversions to an interface with shapes in the one case of a
full-instantiated generic call. So, we do the transformCall() before
installing the shaped-based instantiation. transformCall() works
correctly even in the case of OCALL/FUNCINST.

Fixed two related bugs:
  - Fixed case where we still were not correctly substituting the types
    for a function instantiation.
  - The type substituter needs to copy field flags while substituting in
    tstruct.

Change-Id: I14e960737d6840a75846ede480e6650534ba3af3
Reviewed-on: https://go-review.googlesource.com/c/go/+/340259


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