Moves the generators from SpirvVertexAttributeAliasingTransformer to the
base class, and makes SpirvTransformer use the same functions.

Bug: angleproject:4889
Change-Id: I1f105e2ac764fc7b36c8c75d2cf5f30dec841f97
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2561938Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>