cmd/internal/obj: cleanup linkgetlineFromPos

cmd/internal/obj: cleanup linkgetlineFromPos

Make linkgetlineFromPos and getFileIndexAndLine methods on Link, and
give the former a more descriptive name.

The docs are expanded to make it more clear that these are final
file/line visible in programs.

In getFileSymbolAndLine use ctxt.InnermostPos instead of ctxt.PosTable
direct, which makes it more clear that we want the semantics of
InnermostPos.

Change-Id: I7c3d344dec60407fa54b191be8a09c117cb87dd0
Reviewed-on: https://go-review.googlesource.com/c/go/+/446301
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>