cmd/compile: adjust lineno during import to get Pos right

Binary import sometimes constructs nodes using functions
that use the global lineno for the Position.  This causes
spurious numbers to appear in the assembly and the
debugging output.

Fix (targeted, because late in the cycle): save and restore
lineno around bimport calls known to use lineno-sensitive
functions.

Updates #22600.
(Comment: "This is a weird line to step through")

Change-Id: I9c4094670380609fe4b6696443fb02579521c596
Reviewed-on: https://go-review.googlesource.com/80115


Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>