go/types, types2: better error message for invalid == on type parameters

[ferrmin]

Fixes golang#48712.

Change-Id: I6f214cdfdd1815493f2a04828e8f0097f1d8c124
Reviewed-on: https://go-review.googlesource.com/c/go/+/372734
Trust: Robert Griesemer gri@golang.org
Run-TryBot: Robert Griesemer gri@golang.org
TryBot-Result: Gopher Robot gobot@golang.org
Reviewed-by: Robert Findley rfindley@google.com

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