cmd/compile/internal/types2: fix type inference

Don't let type parameters that are not filled in with concrete
type arguments escape from constraint type inference - such
inferred types are not "real".

While at it, implement a tparamsList.String method for debugging.

Fixes #45548.

Change-Id: I40f13ff7af08d0357a5c66234bfcdd0b7ed5fdd6
Reviewed-on: https://go-review.googlesource.com/c/go/+/311651


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