[dev.typeparams] cmd/compile/internal/syntax: accept embedded type literals

The parser accepted embedded elements but the first term
of an element had to be a ~-term or a type name. This CL
fixes that.

Change-Id: I013b6cdc5963fb228867ca6597f9139db2be7ec5
Reviewed-on: https://go-review.googlesource.com/c/go/+/321109


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