Incorrect offsets when using --hlsl-offsets switch

When using the HLSL offsets mode the following buffer definitions generate offsets that improperly straddle 16-byte boundaries. This means that they are not compatible with Vulkan's relaxed layouts, and I think that they are supposed to be.

```
struct {
float x;
struct { // Alignment 8, Offset 8
float a;
vec2 b; // Offset 4 within the struct => offset 12, and improperly straddles to 20
} y;
};
```
```
struct {
float a; // Offset 0
vec2 b; // Offset 4
float c; // Offset 12
float d; // Offset 16
} x[2]; // Alignment 8 => Array stride of 24
```
`x[1].b` improperly straddles from offset 28 to 36.

Start by reproducing the two buffer-layout cases with the --hlsl-offsets switch and inspect the compiler path that calculates their nested-member and array offsets. Compare the generated offsets with Vulkan relaxed-layout requirements; the work is done when the nested vec2 and x[1].b no longer straddle 16-byte boundaries.

Written by the indexing model from the issue text.