cmd/compile/internal/types2: return an error from Instantiate

Change Instantiate to be a function (not a method) and return an error.
Introduce an ArgumentError type to report information about which type
argument led to an error during verification.

This resolves a few concerns with the current API:
 - The Checker method set was previously just Files. It is somewhat odd
   to add an additional method for instantiation. Passing the checker as
   an argument seems cleaner.
 - pos, posList, and verify were bound together. In cases where no
   verification is required, the call site was somewhat cluttered.
 - Callers will likely want to access structured information about why
   type information is invalid, and also may not have access to position
   information. Returning an argument index solves both these problems;
   if callers want to associate errors with an argument position, they
   can do this via the resulting index.

We may want to make the first argument an opaque environment rather than
a Checker.

Change-Id: I3bc56d205c13d832b538401a4c91d3917c041225
Reviewed-on: https://go-review.googlesource.com/c/go/+/342152


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>