[mlir][dxsa] Add dcl_function_body and dcl_function_table

[asavonic]

DX assembly programs support "virtual" inlining of subroutines by defining a set of functions that might be called for a call site.
The compiler then prepares an inlined specialization for every possible callee. The actual specialization is selected at runtime.

On DXBC level, functions are declared and identified by numbers, grouped into tables, and tables are grouped as interfaces:

// Function table for AmbientLight.
dcl_function_body fb0
dcl_function_table ft0 = { fb0 }

// Function table for DirectionalLight.
dcl_function_body fb1
dcl_function_table ft1 = { fb1 }

// main's MyMaterial parameter.
dcl_interface fp0[1][1] = { ft0, ft1 };

// main shader code

// call AmbientLight or DirectionalLight based on function pointer bound
fcall fp0[0][0]

This patchset adds only dcl_function_body and dcl_function_table. Interfaces are going to be in a separate patch.