It is always legal in Vulkan to read a builtin, particularly
`BuiltInPosition`, even if it weren't written by the previous stage. The
CTS tests that this scenario works in the driver.
Update SPIRV-Cross to pull in a change required for this.
Fixes 8 CTS tests under `dEQP-VK.pipeline.*.no_position`. (Eight other
tests worked solely by accident without this change.)
fafcc4b844