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/cli/cli-interp.c

@@ -108,9 +108,9 @@ should_print_stop_to_console (struct interp *console_interp,
 {
   if ((bpstat_what (tp->control.stop_bpstat).main_action
        == BPSTAT_WHAT_STOP_NOISY)
-      || tp->thread_fsm == NULL
-      || tp->thread_fsm->command_interp == console_interp
-      || !tp->thread_fsm->finished_p ())
+      || tp->thread_fsm () == nullptr
+      || tp->thread_fsm ()->command_interp == console_interp
+      || !tp->thread_fsm ()->finished_p ())
     return 1;
   return 0;
 }