Represents a token that indicates execution on the compilation thread, i.e.
   - we have full access to the (partially mutable) TAST and TcImports data structures
   - compiler execution may result in type provider invocations when resolving types and members
   - we can access various caches in the SourceCodeServices

 Like other execution tokens this should be passed via argument passing and not captured/stored beyond
 the lifetime of stack-based calls. This is not checked, it is a discipline within the compiler code.