[SPIR-V] Forward declaring a function inside another function causes a compiler crash

[Dredhog]

Title

Forward declaring a function inside another function causes a compiler crash.

Functional impact

Can't have forward declarations inside functions. Breaks compatibility with the DXIL backend and backward compatibility with FXC which compile the following example fine.

Minimal repro steps

1.1. Compile forward_decl_issue.txt (seen below) with dxc.exe -T ps_6_0 -E PSMain -spirv forward_decl_issue.txt:

float4 PSMain() : SV_Target
{
  float MulBy2(float f);
  return float4(MulBy2(.25), 1, 0, 1);
}

float MulBy2(float f)
{
    return f*2;
}

Expected result

Compilation succeeds.

Actual result

Compiler crashes on an assertion with the following stack: crash_stack.txt

Further technical details

DXC built from commit 530958e