DXC linker debug output isn't a valid PDB (and doesn't work with PIX)

Description
Linking a shader with -Fd will dump the raw ILDB part from the DXIL container instead of a valid PDB file.

Steps to Reproduce
Create a file named test.hlsl:

RWTexture2D<uint> rwTexResource[] : register(u0, space2400);
[numthreads(8, 8, 1)]
void main(uint3 dtid : SV_DispatchThreadID, uint3 gtid : SV_GroupThreadID, uint3 gid : SV_GroupID, uint gindex : SV_GroupIndex)
{
    rwTexResource[0][dtid.xy] = texResource.Load(dtid.xyz);
}

Run these commands:

dxc.exe -T lib_6_3 -Zi -Qstrip_reflect -Qembed_debug -Fd testc.pdb -Fo test.lib test.hlsl
dxc.exe -link -T lib_6_3 -Zi -Qstrip_reflect -Fd test.pdb -Fo test.bin test.lib

Actual Behavior
testc.pdb (the compiler output) has a valid PDB header.
test.pdb does not.

"PDBs" output this way will also not load in PIX.

Environment

• DXC version dxcompiler.dll: 1.7 - 1.7.2207.3 (e9137cd1d); dxil.dll: 1.7(101.7.2207.10)
• Host Operating System Windows 10

Start by reproducing the issue with test.hlsl and the two dxc.exe commands from the report, then compare the compiler-generated testc.pdb with the linker-generated test.pdb. Investigate the DXC linker path that emits debug output and verify that the resulting file has a valid PDB header and loads in PIX.