This avoids a lot of huge code changes.
Arrays generally cannot be copied in and out of buffers, at least no
compiler frontend seems to do it.
Also avoids a lot of issues surrounding packed vectors and matrices.
Support Workgroup (threadgroup) variables.
Mark if SPIRConstant is used as an array length, since it cannot be specialized.
Resolve specialized array length constants.
Support passing an array to MSL function.
Support emitting GLSL array assignments in MSL via an array copy function.
Support for memory and control barriers.
Struct packing enhancements, including packing nested structs.
Enhancements to replacing illegal MSL variable and function names.
Add Compiler::get_entry_point_name_map() function to retrieve entry point renamings.
Remove CompilerGLSL::clean_func_name() as obsolete.
Fixes to types in bitcast MSL functions.
Add Variant::get_id() member function.
Add CompilerMSL::Options::msl_version option.
Add numerous MSL compute tests.
