GL_EXT_structured_descriptor_heap using Int8 instead of OffsetIdEXT

```glsl
#version 460
#extension GL_EXT_descriptor_heap : require
#extension GL_EXT_structured_descriptor_heap : require
#extension GL_EXT_scalar_block_layout : require

layout(buffer_type, scalar) buffer SSBO_B {
uint x_b;
uint y_b;
};

layout(heap_offset = 64) resourceheap BufferHeap {
SSBO_B buf_b; // starts at offset = bufferDescriptorSize
} bufferHeap;

void main() {
bufferHeap.buf_b.x_b = 2;
}
```

currently generates

```
OpCapability Int8
%uchar = OpTypeInt 8 0
%_runtimearr_uchar = OpTypeRuntimeArray %uchar
```

and not sure why we need `shaderInt8` for this? I would have used `OffsetIdEXT` to set the offset instead making this a strange `uint8_t*` array

cc @ShchchowAMD @Guang-035

Reproduce the supplied GLSL shader with glslang and inspect the generated SPIR-V around the heap buffer representation. Compare the emitted Int8/uchar runtime array and Int8 capability with the extension semantics and the expected OffsetIdEXT approach; done means the behavior is explained and corrected if it is incorrect.

Written by the indexing model from the issue text.