cmd/compile/internal/types2: when type hashing, use placeholders for type parameters
This is a port of CL 363114 from go/types to types2 with a temporary
work-around in tparamIndex to avoid a crash with the unified build
and test/typeparam/setsimp.go.
Change-Id: Id4805385f21c95b461911b246fb47ee278a84ac9
Reviewed-on: https://go-review.googlesource.com/c/go/+/364336
Trust: Robert Griesemer <gri@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>