[WIP] Debug info entry as local

[jkwak-work]

DO NOT REVIEW
WIP

The intention

This commit is to reconstruct the entry-point parameters as local variables.
And then, we will be able to provide the debug information to the shader debuggers easily.

Consider the pre-existing test, tests/spirv/debug-variable-scope.slang,

struct PSIn
{
   float4 pos      : SV_Position;
};

float4 main(PSIn input) : SV_TARGET
{ ... }

Currently the following command removes the entry point variable and each member is inlined to where they are used.

$ build/Debug/bin/slangc.exe -target glsl tests/spirv/debug-variable-scope.slang -stage fragment -entry main -line-directive-mode none

void main()
{
    entryPointParam_main_0 = vec4((texture(sampler2D(testTex_0,testSampler_0), (vec2(uvec4(gl_FragCoord).xy) / 32.0), (-1.0))).xyz, 1.0);
    return;
}

This commit is to reconstruct the entry-point parameter as a local variable so that we can provide Debug info for the entry point parameter,

void main()
{
    /////// ENTRY POINT PARAMETER IS RECONSTRUCTED AS LOCAL VARIABLE
    PSIn input;
    input.pos = gl_FragCoord;
    /////// ENTRY POINT PARAMETER IS RECONSTRUCTED AS LOCAL VARIABLE

    entryPointParam_main_0 = vec4((texture(sampler2D(testTex_0,testSampler_0), (vec2(uvec4(input.pos).xy) / 32.0), (-1.0))).xyz, 1.0);
    return;
}

When the entry-point parameter is an array of struct

One thing to note is that when the entry point is an array of struct, tests/cross-compile/geometry-shader.slang,

struct CoarseVertex
{
    float4 position     : POSITION;
    float3 color        : COLOR;
    uint   id           : ID;
}

void main(
    triangle CoarseVertex coarseVertices[3],
    inout TriangleStream<RasterVertex> outputStream,
    uint primitiveID : SV_PrimitiveID)
{ ... }

Currently it generates the following,

$ build/Debug/bin/slangc.exe -target glsl -stage geometry -entry main tests/cross-compile/geometry-shader.slang -line-directive-mode none

void main()
{
    CoarseVertex_0 _S1 = { coarseVertices_position_0[0], coarseVertices_color_0[0], coarseVertices_id_0[0] };
    CoarseVertex_0 _S2 = { coarseVertices_position_0[1], coarseVertices_color_0[1], coarseVertices_id_0[1] };
    CoarseVertex_0 _S3 = { coarseVertices_position_0[2], coarseVertices_color_0[2], coarseVertices_id_0[2] };
    CoarseVertex_0  _S4[3] = { _S1, _S2, _S3 };

This commit will change it to the following,

void main()
{
    /////// ENTRY POINT PARAMETER IS RECONSTRUCTED AS LOCAL VARIABLE
    CoarseVertex_0 coarseVertices[3];
    coarseVertices[0] = { coarseVertices_position_0[0], coarseVertices_color_0[0], coarseVertices_id_0[0] };
    coarseVertices[1] = { coarseVertices_position_0[1], coarseVertices_color_0[1], coarseVertices_id_0[1] };
    coarseVertices[2] = { coarseVertices_position_0[2], coarseVertices_color_0[2], coarseVertices_id_0[2] };
    /////// ENTRY POINT PARAMETER IS RECONSTRUCTED AS LOCAL VARIABLE

    CoarseVertex_0 _S1 = coarseVertices[0];
    CoarseVertex_0 _S2 = coarseVertices[1];
    CoarseVertex_0 _S3 = coarseVertices[2];

The tricky part is that the system build-in variables are in a form of array for each semantic.
And the user expect to get the debug information in a form of "array-of-struct".
The implementation maps each member variable to its corresponding global variable with DebugValue.