go/types: ensure that constructed type parameters are immutable

TypeParam.iface may mutate TypeParam.bound in the event that the type
parameter bound is not an interface.

Ensure that iface() is called before the type-checking pass returns, and
before NewTypeParam or TypeParam.SetConstraint exits.

Fixes #49788

Change-Id: I72279acf5f0223161671c04887bc2c3df4158927
Reviewed-on: https://go-review.googlesource.com/c/go/+/367614


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>