[SPIR-V] GetAttributeAtVertex(0,0) fails validation

[sudonatalie]

Description
Follow-up from #6041 (comment)

This invalid usage of GetAttributeAtVertex fails at validation rather than providing a useful error message.

Steps to Reproduce

dxc -T ps_6_0 -spirv

float main() : SV_Target
{
  return GetAttributeAtVertex(0, 0);
}

Actual Behavior
SPIR-V compilation fails with a validation error message:

fatal error: generated SPIR-V is invalid: The Base <id> '7[%int_0]' in OpAccessChain instruction must be a pointer.
  %14 = OpAccessChain %_ptr_Function_int %int_0 %uint_0

DXIL fails with:

<source>:6:10: error: attribute evaluation can only be done on values taken directly from inputs.
  return GetAttributeAtVertex(0, 0);
         ^

The SPIR-V backend should produce a similarly helpful error message.

[sudonatalie]

Related: microsoft/hlsl-specs#181