cmd/compile: fix panic in DWARF-gen handling obfuscated code

DWARF generation uses variable source positions (file/line/col) as a
way to uniquely identify locals and parameters, as part of the process
of matching up post-optimization variables with the corresponding
pre-optimization versions (since the DWARF needs to be in terms of the
original source constructs).

This strategy can run into problems when compiling obfuscated or
machine-generated code, where you can in some circumstances wind up
with two local variables that appear to have the same name, file,
line, and column. This patch changes DWARF generation to skip over
such duplicates as opposed to issuing a fatal error (if an
obfuscation tool is in use, it is unlikely that a human being will be
able to make much sense of DWARF info in any case).

Fixes #44378.

Change-Id: I198022d184701aa9ec3dce42c005d29b72d2e321
Reviewed-on: https://go-review.googlesource.com/c/go/+/294289


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