Possible zero array size from spec-const expression not disallowed

Observe:

**broken.comp**:
```glsl
#version 450
#extension GL_ARB_compute_shader : enable

layout(constant_id=0) const uint foo = 0u;
shared float bar[foo];

void main() {}
```

**fixed.comp**:
```glsl
#version 450
#extension GL_ARB_compute_shader : enable

layout(constant_id=0) const uint foo = 0u;
shared float bar[foo + 0u];

void main() {}
```

Output:
```
$ glslangValidator --target-env vulkan1.2 broken.comp
broken.comp
ERROR: broken.comp:5: '' : array size must be a positive integer
ERROR: broken.comp:5: '' : compilation terminated
ERROR: 2 compilation errors. No code generated.
```

```
$ glslangValidator --target-env vulkan1.2 fixed.comp
fixed.comp
```

Why does adding a `+ 0u` to the array size spec constant make it succeed compiling? Incidentally, the shader works fine at runtime - assuming the correct (nonzero) array size is substituted in using a `VkSpecializationInfo` at pipeline creation time.

Reproduce the difference between broken.comp and fixed.comp with glslangValidator --target-env vulkan1.2. Trace how the compiler validates the shared array size when it is a specialization-constant expression, then compare that path with the expression containing + 0u. Done means the behavior is explained and zero-valued defaults are handled consistently with specialization at pipeline creation.

Written by the indexing model from the issue text.