go/parser: (x.(type)) is not a valid type switch guard

[griesemer]

The parser appears to accept (x.(type)) as a type switch guard. Investigate.
See also issue #4470.

[griesemer]

Comment 1:

The parser should continue to accept
switch (x.(type)) { ... }
in a type switch for now so that incorrect code is correctly rewritten by gofmt. The
form:
switch t := (x.(type)) { ... }
was never accepted, so leave it alone.
See CL 6867062 for a test case.

Status changed to WorkingAsIntended.