Most of the AST transforms are written as a workaround to an issue that
affects a single backend.  This change identifies such transforms and
organizes them by backend.  They are then only built if the respective
backend is.

Additionally, about half of the GL transforms are due to mac
workarounds, including the large RewriteRowMajorMatrices transform.
Mac-specific workarounds are additionally only built on said platform.

This change reduces the ANGLE binary size:

- 106KB in a Vulkan-only build on Linux
- 27KB in a GL-only build on Android (60KB on Linux)

Bug: chromium:1084580
Bug: chromium:1161513
Change-Id: I64b334332c0d4f848756c6538af0d8d96864c7e9
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2601346
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>