MSL: Ignore duplicate builtin vertex attributes.
MSL: Ignore duplicate builtin vertex attributes.

These are often arrayed builtins, which MSL maps to more than one
attribute. SPIRV-Cross automatically assigns succeeding addresses to
arrayed attributes, so we really only need the first one. This of course
assumes that the inputs are sorted by location.

KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross