SPIR-V: `SubpassInput` should default to `float4` if explicit type parameter is not provided

[chaoticbob]

Slang currently expects an explicit type parameter for SubpassInput, this is contrary to the expected usage of SubpassInput from DXC (where it originated I'm guessing) and results in the following compile error:

shader.hlsl(3): error 30400: generic type '<DeclRef<GenericDecl>>' used without argument
SubpassInput gSubpassInputs[32];

According to DXC's SPIR-V docs: In the above, T is a scalar or vector type. If omitted, it will defaults to float4.

Shader

SubpassInput gSubpassInputs[32];

float4 main(uint index : A) : SV_TARGET{
    float4 v2 = gSubpassInputs[NonUniformResourceIndex(index)].SubpassLoad();
    return v2;
}