Apparently I never actually understood the new file table in Go
object files. The PC value stream actually encodes the file index
in the per-CU table. I thought it was indexing into a per-function
table, which then contains index to the per-CU table. Remove the
extra indirection.

Change-Id: I0aea5629f7b3888ebe3a04fea437aa15ce89519e
Reviewed-on: https://go-review.googlesource.com/c/go/+/262779


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>