[windows] Host a symbol server for Swift toolchain releases

[charles-zablit]

When debugging the Swift toolchain on Windows, it's time consuming to build from scratch with -DebugInfo enabled to be able to symbolicate a backtrace. Making debug symbols available to download for releases would be a great step to help diagnose issues faster.

On Windows, debuggers can use Symbol servers to fetch debug symbols and symbolicate crash logs and allow source breakpoints resolution.

In order to setup a symbol server for the toolchain, we need to do 2 things:

• Configure CMake in all subprojects to install PDB files in the correct directory when building.
• Create a script which lays out the PDB files in the expected structure, using symstore.exe.
• Make the directory available on object storage.

cc: @shahmishal @al45tair @carlpeto @compnerd