cmd/compile: fix libfuzzer instrumentation line number
Set a reasonable starting line number before processing the body of
the function in the order pass.
We update base.Pos each time we process a node, but some of the
libfuzzer instrumentation is added before we process any node, so the
base.Pos used is junk.
Fixes #53688
Change-Id: I3654b805eabb8866a9a1574845ef4ff062797319
Reviewed-on: https://go-review.googlesource.com/c/go/+/416654 Reviewed-by: David Chase <drchase@google.com> Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>