[release-branch.go1.12] cmd/compile: better handling for PAUTOHEAP in DWARF inline gen

When generating DWARF inline info records, the post-SSA code looks
through the original "pre-inline" dcl list for the function so as to
handle situations where formal params are promoted or optimized away.
This code was not properly handling the case where an output parameter
was promoted to the heap -- in this case the param node is converted
in place from class PPARAMOUT to class PAUTOHEAP. This caused
inconsistencies later on, since the variable entry in the abstract
subprogram DIE wound up as a local and not an output parameter.

Updates #30908.
Fixes #31028.

Change-Id: Ia70b89f0cf7f9b16246d95df17ad6e307228b8c7
Reviewed-on: https://go-review.googlesource.com/c/go/+/168818


Reviewed-by: Cherry Zhang <cherryyz@google.com>
(cherry picked from commit 68a98d52)
Reviewed-on: https://go-review.googlesource.com/c/go/+/169417
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>