Normalize TRY_CATCH exception handling block

This normalizes some exception catch blocks that check for ex.reason
to look like this:

~~~
  volatile gdb_exception ex;

  TRY_CATCH (ex, RETURN_MASK_ALL)
    {
      ...
    }
  if (ex.reason < 0)
    {
      ...
    }
~~~

This is a preparation step for running a script that converts all
TRY_CATCH uses to look like this instead:

~~~
  TRY
    {
      ...
    }
  CATCH (ex, RETURN_MASK_ALL)
    {
      ...
    }
  END_CATCH
~~~

The motivation for that change is being able to reimplent TRY/CATCH in
terms of C++ try/catch.

This commit makes it so that:

 - no condition other than ex.reason < 0 is checked in the if
   predicate

 - there's no "else" block to check whether no exception was caught

 - there's no code between the TRY_CATCH (TRY) block and the
   'if (ex.reason < 0)' block (CATCH).

 - the exception object is no longer referred to outside the if/catch
   block.  Note the local volatile exception objects that are
   currently defined inside functions that use TRY_CATCH will
   disappear.  In cases it's more convenient to still refer to the
   exception outside the catch block, a new non-volatile local is
   added and copy to that object is made within the catch block.

The following patches should make this all clearer.

gdb/ChangeLog:
2015-03-07  Pedro Alves  <palves@redhat.com>

	* amd64-tdep.c (amd64_frame_cache, amd64_sigtramp_frame_cache)
	(amd64_epilogue_frame_cache): Normal exception handling code.
	* break-catch-throw.c (check_status_exception_catchpoint)
	(re_set_exception_catchpoint): Ditto.
	* cli/cli-interp.c (safe_execute_command):
	* cli/cli-script.c (script_from_file): Ditto.
	* compile/compile-c-symbols.c (generate_c_for_for_one_variable):
	Ditto.
	* compile/compile-object-run.c (compile_object_run): Ditto.
	* cp-abi.c (baseclass_offset): Ditto.
	* cp-valprint.c (cp_print_value): Ditto.
	* exceptions.c (catch_exceptions_with_msg):
	* frame-unwind.c (frame_unwind_try_unwinder): Ditto.
	* frame.c (get_frame_address_in_block_if_available): Ditto.
	* i386-tdep.c (i386_frame_cache, i386_epilogue_frame_cache)
	(i386_sigtramp_frame_cache): Ditto.
	* infcmd.c (post_create_inferior): Ditto.
	* linespec.c (parse_linespec, find_linespec_symbols):
	* p-valprint.c (pascal_object_print_value): Ditto.
	* parse.c (parse_expression_for_completion): Ditto.
	* python/py-finishbreakpoint.c (bpfinishpy_init): Ditto.
	* remote.c (remote_get_noisy_reply): Ditto.
	* s390-linux-tdep.c (s390_frame_unwind_cache): Ditto.
	* solib-svr4.c (solib_svr4_r_map): Ditto.

gdb/python/py-finishbreakpoint.c

@@ -269,8 +269,13 @@ bpfinishpy_init (PyObject *self, PyObject *args, PyObject *kwargs)
             }
         }
     }
-  if (except.reason < 0
-      || !self_bpfinish->return_type || !self_bpfinish->function_value)
+  if (except.reason < 0)
+    {
+      /* Just swallow.  Either the return type or the function value
+	 remain NULL.  */
+    }
+
+  if (self_bpfinish->return_type == NULL || self_bpfinish->function_value == NULL)
     {
       /* Won't be able to compute return value.  */
       Py_XDECREF (self_bpfinish->return_type);