go/parser: fix parsing of array or slice constraint types

Now that we allow eliding 'interface' from constraint types, we need to
be a bit more careful about not consuming a '[' when parsing the next
expression after "type T [". We want to check if the next expression is
an identifier not followed by ']', in which case we're in a generic
type, but need to avoid parsing index or slice expressions. Such
expressions aren't valid array lengths because these expressions are
never constant, so when encountering a following '[' we can instead
assume that this is a type parameter field with array or slice type
constraint.

Test cases are added for the related issues #49174 and #49175, along
with a flag to enable tracing error tests.

For #49174
For #49175

Change-Id: I0476ef20c4c134ac537118272f20caaf123ee70e
Reviewed-on: https://go-review.googlesource.com/c/go/+/359134


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