gdb: make thread_info::m_thread_fsm a std::unique_ptr

While working on function calls, I realized that the thread_fsm member
of struct thread_info is a raw pointer to a resource it owns.  This
commit changes the type of the thread_fsm member to a std::unique_ptr in
order to signify this ownership relationship and slightly ease resource
management (no need to manually call delete).

To ensure consistent use, the field is made a private member
(m_thread_fsm).  The setter method (set_thread_fsm) can then check
that it is incorrect to associate a FSM to a thread_info object if
another one is already in place.  This is ensured by an assertion.

The function run_inferior_call takes an argument as a pointer to a
call_thread_fsm and installs it in it in a thread_info instance.  Also
change this function's signature to accept a unique_ptr in order to
signify that the ownership of the call_thread_fsm is transferred during
the call.

No user visible change expected after this commit.

Tested on x86_64-linux with no regression observed.

Change-Id: Ia1224f72a4afa247801ce6650ce82f90224a9ae8

gdb/breakpoint.c

@@ -10830,8 +10830,10 @@ until_break_command (const char *arg, int from_tty, int anywhere)
       breakpoints.emplace_back (std::move (location_breakpoint));
     }
 
-  tp->thread_fsm = new until_break_fsm (command_interp (), tp->global_num,
-					std::move (breakpoints));
+  tp->set_thread_fsm
+    (std::unique_ptr<thread_fsm>
+     (new until_break_fsm (command_interp (), tp->global_num,
+			   std::move (breakpoints))));
 
   if (lj_deleter)
     lj_deleter->release ();