[lldb/Plugins] Fix ScriptedThread IndexID reporting

When listing all the Scripted Threads of a ScriptedProcess, we can see that all
have the thread index set to 1. This is caused by the lldb_private::Thread
constructor, which sets the m_index_id member using the provided thread id `tid`.

Because the call to the super constructor is done before instantiating
the `ScriptedThreadInterface`, lldb can't fetch the thread id from the
script instance, so it uses `LLDB_INVALID_THREAD_ID` instead.

To mitigate this, this patch takes advantage of the `ScriptedThread::Create`
fallible constructor idiom to defer calling the `ScriptedThread` constructor
(and the `Thread` super constructor with it), until we can fetch a valid
thread id `tid` from the `ScriptedThreadInterface`.

rdar://87432065

Differential Revision: https://reviews.llvm.org/D117076



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>