Autotmp filtering was too aggressive and excluded types
necessary to make debuggers work properly.  Restore the
"late filter" in dwarf.go based on names to exclude autotmps,
and remove the "early filter" in pgen.go based on how the
name was introduced.  However, the updated naming scheme
with a dot prefix is retained to prevent accidental clashes
with legal Go identifier names.

Includes test (grouped with runtime gdb tests),
verified to fail without the fix.

Updates #17644.
Fixes #17830.

Change-Id: I7ec3f7230083889660236e5f6bc77ba5fe434e93
Reviewed-on: https://go-review.googlesource.com/33233
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>