go/types, types2: types in type switch cases must be instantiated

We already have a function that does all the right checks and it's called
varType. The only reason it wasn't used for type switch cases was that we
also have to accept the nil value. That was handled with typeOrNil. But
that function (typeOrNil) was only used for this specific purpose and I long
wished to get rid of it. It turns out that there's only one way to write the
untyped value nil, which is to actually write "nil" (maybe with parentheses).
So looking for that turned out to be simpler than using typeOrNil.

The new code does exactly that, and now we can just use varType and delete
typeOrNil. With this, there is now less code (excluding the test) and the code
is simpler and more correct.

Fixes #48008.

Change-Id: I8f2d80e61ae663c886924909f22bbfa634e7779c
Reviewed-on: https://go-review.googlesource.com/c/go/+/345790


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>