Add warning if Forward Declaration uses layout qualifiers

[spencer-lunarg]

closes #3774

Now something like

#version 450
#extension GL_EXT_buffer_reference : enable

layout(buffer_reference, std140) buffer StructB;
layout(buffer_reference) buffer StructB {
    vec4 x;
};

void main() {}

will give a warning of

WARNING: test.comp:4: 'StructB' : the packing layout (scalar, std430, etc) is ignored when defined in forward declaration

[arcady-lunarg]

Could you add a test for this? There's an existing spv.bufferhandle_Error.frag test file you can just add a couple of lines to.