Windows: Add srcsrv to pdb, mapping source file paths to github URLs

The PDB files which are uploaded for releases on Windows currently do not contain a "srcsrv" stream. This means that, if they are used for debugging in Visual Studio, the source code will not be loaded automatically, because the PDB files will refer to absolute file paths from the build machine.

A srcsrv stream can map these absolute file paths to URLs, so that Visual Studio can download the source from a source server. For projects which are publicly hosted on github, raw.githubusercontent.com is a fine source server.

Here's a tutorial for how to put the right information into the PDB: https://gist.github.com/baldurk/c6feb31b0305125c6d1a#source-indexing

Start by tracing how Windows release PDB files are produced and uploaded in cargo-dist, then read the linked source-indexing tutorial. Done means released Windows PDBs contain a srcsrv stream that maps build-machine paths to raw.githubusercontent.com URLs so Visual Studio can retrieve the source automatically.

Written by the indexing model from the issue text.