[Mono.Debugging.Soft] Make sure to lock the SourceLinkCache while accessing it
[Mono.Debugging.Soft] Make sure to lock the SourceLinkCache while accessing it

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1093441/

mono/debugger-libs,mono/debugger-libs,Unity-Technologies/debugger-libs,Unity-Technologies/debugger-libs