- Previously all default shader files were included in one file named
  master_source.metal.
- Now they will be compiled separately and link together to generate
  binary shader.
- Also generate a debug binary variant that will be used in debug build.

Bug: angleproject:2634
Change-Id: Ic040835c4a729666e73afeba52f5be839b632396
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2281784
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>