Blender Git Loki

Cycles: use OpenCL C 2.0 if available, to improve performance for AMD

Tested with AMD Radeon Pro WX 9100, where it brings performance back to 2.80
level, and combined with recent changes is about 2-15% faster than 2.80 in
our benchmark scenes.

This somehow appears to specifically address the issue where adding more shader
nodes leads to slower runtime. I found no additional speedup by applying this
to change to 2.80 or removing the new shader node code.

Ref T71479

Patch by Jeroen Bakker.

Differential Revision: https://developer.blender.org/D6252