CI: build LLVM Windows nightly builds with PDB files

The PDB files will be inside the -debug.7z package.

The PDB source mapping will be the '/vlc' folder to '/'.

In Visual Studio you can map to your local source folder with:

Solution > Properties > Debug Source Files > "s:/sources/vlc"

In Visual Studio Code you can map to your local source folder with the "sourceFileMap" variable in the launch configuration https://code.visualstudio.com/docs/cpp/launch-json-reference#_sourcefilemap:

"sourceFileMap": { "/": "s:/sources/vlc" }