Generator can't access WinMD files anymore

[Boddlnagg]

Starting with Windows 10 1903 the C#-based generator can't access the metadata files in C:\Windows\System32\WinMetadata (at least when run from inside VS), because of the problem described in https://developercommunity.visualstudio.com/content/problem/446490/visual-studio-cant-see-winmetadatadirectory-in-cwi.html (i.e. it's actually looking in C:\WINDOWS\SysWOW64\WinMetadata and that doesn't exist anymore).

The currently work-in-progress Rust generator (#50) doesn't have this problem, but we should probably still look for these files in another location, because (as a comment in the above link says)

WinMetadata directory was only intended for use by the OS

which makes sense because it's not versioned or anything.