Instead of generating one CodeStubAssembler-like class per namespace,
Torque-generated macros are now free-standing functions not included
from CSA code, and explicitly exported macros become part of the new
TorqueGeneratedExportedMacrosAssembler, which CodeStubAssembler
inherits from, thus making them available to all CSA code.

Structs are now defined in a new header csa-types-tq.h as free-standing
types with the prefix "TorqueStruct".

This is a preparation for generating per Torque-file instead of per
namespace.

Change-Id: I60fadc493a63f85d1d340768ec6f11ae47be0cb8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1628787
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61865}