No longer use inheritance to associate Torque-generated assemblers
with corresponding CSA subclasses. Instead, all references to CSA
and CSA-derived assemblers are now explicitly qualified, by generating
a short-lived assembler instance in-place. As a consequence, Torque
files have to mention the assembler external macros live in.
The CodeStubAssembler is the default for this and can be omitted.
As a drive-by cleanup, also distinguish between names that are emitted
in C++ and names that are intended to be read in error messages. This
is relevant for generic instantiations, where the generated names are
rather unreadably mangled.

As a follow-up, it will be easy to allow for qualified access to
different modules, thus implementing full namespace semantics for
modules.

Bug: v8:7793
Change-Id: Ie6f1b6b549b510fb49be2442393d898d5f130950
Reviewed-on: https://chromium-review.googlesource.com/c/1309636
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Daniel Clifford <danno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57235}