go_spec: use PrimaryExpr as type switch expression
Without this change, the spec parses <-c.(type) as (<-c).(type)
but parses <-c.(int) as <-(c.(int)).
With this change, the former parses as <-(c.(type)).
All three parsers already implement this rule, because
they look for the "type" word during parsing of a standard
type assertion. This change merely brings the
spec in line with the implementations.