Validator should check that push constants, uniforms, ssbos are structs

In the Vulkan spec Chapter 14 it states that push constants, uniform blocks and storage blocks should all be OpTypeStruct. Or an array of structs for uniform/storage blocks.

Currently though this isn't checked, e.g. as in this shader:

```
OpCapability Shader
OpMemoryModel Logical Simple
OpEntryPoint Vertex %main "main" %gl_Position
OpName %main "main"
OpDecorate %gl_Position BuiltIn Position
OpDecorate %uniform_var Binding 0
OpDecorate %uniform_var DescriptorSet 0
%void = OpTypeVoid
%float = OpTypeFloat 32
%v4float = OpTypeVector %float 4
%_ptr_Uniform_v4float = OpTypePointer Uniform %v4float
%_ptr_Output_v4float = OpTypePointer Output %v4float
%gl_Position = OpVariable %_ptr_Output_v4float Output

; this definition
%uniform_var = OpVariable %_ptr_Uniform_v4float Uniform

%25 = OpTypeFunction %void
%main = OpFunction %void None %25
%83 = OpLabel
%51 = OpLoad %v4float %uniform_var
OpStore %gl_Position %51
OpReturn
OpFunctionEnd
```

No files, tests, or entry points are named. Start by locating the Vulkan validator checks for OpVariable declarations in the SPIR-V Tools source, then compare their behavior with the Chapter 14 type rules. Done means invalid non-struct push constants, uniform blocks, and storage blocks are rejected, while permitted arrays of structs remain accepted.

Written by the indexing model from the issue text.