cmd/compile/internal/dwarfgen: use src.Pos.Rel{Filename,Line,Col} consistently

It appears that this code predates golang.org/cl/96535, which added
RelCol to support /*line*/ directives.

Change-Id: Ib79cebc1be53af706e84e8799eeea81ef8c81c8b
Reviewed-on: https://go-review.googlesource.com/c/go/+/343430


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>