Migrate SwizzleComponent to be shared between SkSL::Swizzle and the DSL.

The SwizzleComponent enum would be useful outside of the DSL. This CL
creates a SkSL::SwizzleComponent namespace and has the DSL inject that
namespace into SkSL::dsl via using-directive. Future CLs will use values
from this enum within SkSL::Swizzle itself.

Users expect that `using namespace SkSL::dsl;` will allow statements
like `Swizzle(var, X, Y)` to work normally without any extra
qualifications, using-declarations or #includes, so this is a little
more involved than forward-declaring an enum would be normally.

Change-Id: Id4e2f109d01e465c315c6278f3d4b87027988c86
Bug: skia:11342
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/372018
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
4 files changed