MSL: dynamic/runtime array conversion

We're trying to convert a runtime array from GLSL -> SPIRV to MSL via SPIRV cross.
The conversion works on Windows, if we look at the GLSL decompiled SPIRV via RenderDoc.
On Mac, we get a statically sized array:

Here's the GLSL:

```
layout(set = 0, binding = 3, std430) readonly buffer boneMatrices
{
vec4 _boneMatrices[];
} _216;
```

Here's the converted MSL:

```
struct boneMatrices
{
float4 _boneMatrices[1];
};
```

The SPIRV looks correct as well:

```
%6 = OpTypeFloat 32
%27 = OpTypeVector %6 4
%213 = OpTypeRuntimeArray %27
%214 = OpTypeStruct %213
%215 = OpTypePointer StorageBuffer %214
%216 = OpVariable %215 StorageBuffer
```

We're using MoltenVK to do the translation, any tips would be appreciated in how we can facilitate the proper conversion.

Thanks.

Start with the GLSL runtime-array example and the corresponding SPIR-V declarations in the issue, then investigate how SPIR-V is converted to MSL through MoltenVK. Reproduce the conversion and determine why the MSL output uses a statically sized array; done means preserving the runtime-array representation in the generated MSL.

Written by the indexing model from the issue text.