[dev.typeparams] cmd/compile/internal/types2: tweak anonymous parameter position

When declaring anonymous parameters, use the syntax.Field's Pos
directly rather than its Type field's Pos. When the type expression is
a qualified identifier (i.e., SelectorExpr), its Pos returns the
position of the dot, whereas we typically declare the anonymous
parameter at the starting position of the type. (We could equivalently
use syntax.StartPos(field.Type), but we already have this as
field.Pos().)

Change-Id: If6ac9635b6e9c2b75a1989d5893a78e0b21cba88
Reviewed-on: https://go-review.googlesource.com/c/go/+/320611


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