During DWARF processing, keep track of the go type symbols for types
directly or indirectly referenced by auto variables in a function,
and add a set of dummy R_USETYPE relocations to the function's DWARF
subprogram DIE symbol.

This change is not useful on its own, but is part of a series of
changes intended to clean up handling of autom's in the compiler
and linker.

Updates #34554.

Change-Id: I974afa9b7092aa5dba808f74e00aa931249d6fe9
Reviewed-on: https://go-review.googlesource.com/c/go/+/197497
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>