cmd/link: move type name mangling after deadcode elimination

Moves type name mangling after deadcode elimination. The motivation for
doing this is to create a space between deadcode elimination and type name
mangling where DWARF generation for types and variables can exist, to fix
issue #23733.

Change-Id: I9db8ecc0f4efe3df6c1e4025f02642fd452f9a39
Reviewed-on: https://go-review.googlesource.com/111236


Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>