[dev.typeparams] go/types: unify methods in missingMethod

Unify methods in Checker.missingMethod. This code was accidentally
dropped from the merge, while dropping support for method type
parameters, but is needed for checking implementations of generic
interfaces.

Put the logic back, including checks that are only needed for method
type parameters. It makes the code no simpler to assume that method type
parameters are disallowed, and we have checks elsewhere that produce
errors for methods with type parameters.

Change-Id: I91f0c9d3e04537fdb9f7ae23a4ce4cec9f1da10e
Reviewed-on: https://go-review.googlesource.com/c/go/+/284252


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