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.
We used to use the Binding decoration for this, but this method is
hopelessly broken. If no explicit MSL resource remapping exists, we
remap automatically in a manner which should always "just work".
This only affects the builtin when it is used, and not when it's passed
to a function. It's a lot cleaner than the way I was doing it before.
Remove the `to_expression()` hack.
In SPIR-V, builtin integral vectors can be either signed or unsigned,
but in MSL they're always unsigned. Unfortunately, the MSL spec forbids
implicit conversions between vector types--even if the corresponding
scalar types would implicitly convert. If you try, the result is a
cryptic error message such as:
```
program_source:37:60: error: cannot convert between vector values of different size ('int4' (aka 'vector_int4') and 'vector_uint4' (vector of 4 'unsigned int' values))
float4 r3 = as_type<float4>((as_type<int4>(r0) * gl_LocalInvocationID.xyyy) + as_type<int4>(r2));
~~~~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~
```
Therefore, uses of these builtins must be explicitly cast, since the
rest of the binary likely assumes that the builtin is of its declared
type.
