[MSL] remap_variable_type_name is largely absent from MSL code emission.

`set_variable_type_remap_callback` does not work on a MSL compiler because `remap_variable_type_name` is largely absent in spriv_msl.cpp.

Two examples of where it's absent:

In `void CompilerMSL::entry_point_args_discrete_descriptors(string &ep_args)`

The image type emission should look more like this:
```
case SPIRType::Image:
{
if (!ep_args.empty())
ep_args += ", ";

// Use Metal's native frame-buffer fetch API for subpass inputs.
const auto &basetype = get(var.basetype);
if (!type_is_msl_framebuffer_fetch(basetype))
{
auto theTypeName = image_type_glsl(type, var_id);
remap_variable_type_name(type, r.name, theTypeName);
ep_args += theTypeName + " " + r.name;
if (r.plane > 0)
ep_args += join(plane_name_suffix, r.plane);
ep_args += " [[texture(" + convert_to_string(r.index) + ")";
if (interlocked_resources.count(var_id))
ep_args += ", raster_order_group(0)";
ep_args += "]]";
}
```

In `string CompilerMSL::to_struct_member(const SPIRType &type, uint32_t member_type_id, uint32_t index, const string &qualifier)`

The last few lines of that method should look more like this:
```
string typeString = type_to_glsl(*declared_type, orig_id);
string memberName = to_member_name(type, index);

remap_variable_type_name(*declared_type, memberName, typeString);
auto result = join(pack_pfx,typeString, " ", qualifier, memberName,
member_attribute_qualifier(type, index), array_type, ";");

is_using_builtin_array = false;
return result;
```


The call to the remap callback is largely absent from whenever CompilerMSL overrides CompilerGLSL.

Start in spriv_msl.cpp with CompilerMSL::entry_point_args_discrete_descriptors and CompilerMSL::to_struct_member, then compare the corresponding CompilerGLSL implementations and inspect set_variable_type_remap_callback. Done means the remap_variable_type_name callback is applied in the identified MSL emission paths and the resulting code matches the issue's examples.

Written by the indexing model from the issue text.