[lldb] Simplify the is_finalized logic in process and make it thread safe.

This is a speculative fix when looking at the finalization code in
Process. It tackles the following issues:

 - Adds synchronization to prevent races between threads.
 - Marks the process as finalized/invalid as soon as Finalize is called
   rather than at the end.
 - Simplifies the code by using only a single instance variable to track
   finalization.

Differential revision: https://reviews.llvm.org/D93479