A flag is added to the translator to disable generation of Bresenham
line raster emulation code when the Bresenham line raster Vulkan
extension is present.  This is primarily for the sake of conversion of
line raster emulation condition to specialization constant:

- Avoid dead SPIR-V code in every shader
- Avoid ANGLEUniforms being active in every shader stage even if it's
  not used anywhere but in line raster emulation.
- Optimize SPIR-V transformations by both having fewer instructions to
  iterate through, and to avoid generating line raster patches.
- Reduce the severity of anglebug.com/4251 where the location assignment
  of ANGLEPosition can incorrectly overlap a varying array or struct, by
  making only platforms without Bresenham extension afflicted by the
  bug.

Bug: angleproject:3394
Change-Id: Ic0ae6ce0392b4eae0cc79cb94bbcd0805b276a31
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1986379Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>