Fork CPPCodeGenerator into a DSL-based version.

The long term goal is for the DSLCPPCodeGenerator to replace the
CPPCodeGenerator entirely, but we will need both to coexist while DSL is
still under development.

Currently, the DSLCPPCodeGenerator is cloned from CPPCodeGenerator and
emits almost exactly the same code (it adds a comment at the top to
distinguish its output). Its output will change in followup CLs.

This CL also allows skslc to recognize the `_dsl.cpp` output suffix and
generate code using DSLCPPCodeGenerator instead of CPPCodeGenerator.
This allows test DSL FPs to be created for inspection.

Change-Id: If5136279c307ea53a9df3a292caa18344c1eb259
Bug: skia:11854
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/396096
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
12 files changed