Subsequent stages can legally attempt to read from these variables,
which causes compilation failure.
Always make sure we emit user outputs in vertex shaders if they are
active in the entry point.
This is necessary to avoid invalid output because of how implicit
dependencies on builtins work.
For example, the fixup for `BuiltInSubgroupEqMask` initializes the
variable based on `builtin_subgroup_invocation_id_id`, a field storing
the ID for a variable with decoration `BuiltInSubgroupLocalInvocationId`.
This could be either a variable that already exists in the input
(spirv_msl.cpp:300) or, if necessary, a newly created one
(spirv_msl.cpp:621). In both cases, though,
`builtin_subgroup_invocation_id_id` is only set under the condition
`need_subgroup_mask || needs_subgroup_invocation_id`.
`need_subgroup_mask` is true if any of the `BuiltInSubgroupXXMask` are
set in `active_input_builtins`.
Normally, if the program contains `BuiltInSubgroupEqMask`,
`Compiler::ActiveBuiltinHandler` will set it in `active_input_builtins`.
But this only happens if the variable is actually used, whereas
`fix_up_shader_inputs_outputs` loops over all variables in the program
regardless of whether they're used.
If `BuiltInSubgroupEqMask` is not used,
`builtin_subgroup_invocation_id_id` is never set, but before this patch
the fixup hook would try to use it anyway, producing MSL that references
a nonexistent variable named `_0`.
Avoid this by changing `fix_up_shader_inputs_outputs` to skip builtins
which are not set in `active_input_builtins` or
`active_output_builtins`. And add a test case.
MSL does not support value semantics for arrays (sigh), so we need to
force constant references and deal with copies if we have a different
address space than what we end up guessing.
