[dev.typeparams] cmd/compile: use types2.Constraint() rather than types2.Bound()

types2.Constraint() returns the top-level constraint type, including any
unions or other interface elements. Because of that, we needed to
add/fix some code in the type substituter and generic type instantiater
in the importer to deal with unions and non-method members of an
interface. Also, NewUnion was not correctly setting the HasTParam flag.

I also added a better error message when a symbol is not found in
(*deadcodePass).decodeIfaceMethod().

Change-Id: Id3668dc596dce63690fa05a9e5e42295b5e2bbb5
Reviewed-on: https://go-review.googlesource.com/c/go/+/340670


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