Validator does not check for offset through decorations groups.

When I run the code below through the validator, it says

> error: line 18: Structure id 6 decorated as Block must be explicitly laid out with Offset decorations.
%_struct_6 = OpTypeStruct %float %float %float

However, the offset are there through group decorations.

```
OpCapability Shader
OpMemoryModel Logical GLSL450
OpEntryPoint Vertex %1 "main"
OpSource HLSL 600
%2 = OpDecorationGroup
%3 = OpDecorationGroup
%4 = OpDecorationGroup
OpDecorate %5 DescriptorSet 0
OpDecorate %5 Binding 0
OpDecorate %2 Offset 0
OpDecorate %3 Offset 4
OpDecorate %4 Offset 8
OpDecorate %6 Block
OpGroupMemberDecorate %2 %6 0
OpGroupMemberDecorate %3 %6 1
OpGroupMemberDecorate %4 %6 2
%7 = OpTypeFloat 32
%6 = OpTypeStruct %7 %7 %7
%8 = OpTypePointer Uniform %6
%9 = OpTypeVoid
%10 = OpTypeFunction %9
%5 = OpVariable %8 Uniform
%1 = OpFunction %9 None %10
%11 = OpLabel
OpReturn
OpFunctionEnd
```

Start by running the provided SPIR-V module through the validator and tracing how OpGroupMemberDecorate is handled when checking the Block structure. Verify whether the grouped Offset decorations are considered for each member; done means the example validates without the reported missing-offset error and coverage exists for this case.

Written by the indexing model from the issue text.