go/parser: disallow parenthesizing embedded types in structs
This was never permitted in Go but the flexibility to do so
was introduced through the generics prototype code where we
experimented with parentheses to enclose type parameters.
Restore original (pre-generics) behavior.
Fixes #51655.
Change-Id: Ia7a4b2e393e0214a70e840c8663cf4474c5c754b
Reviewed-on: https://go-review.googlesource.com/c/go/+/424694
Run-TryBot: Robert Griesemer <gri@google.com> Reviewed-by: Robert Griesemer <gri@google.com> Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>