[opt-viewer] Flush stdout after progress update

Without this, there was no progress shown during parsing but only during
rendering on macOS.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@308000 91177308-0d34-0410-b5e6-96231b3b80d8

tools/opt-viewer/optpmap.py

@@ -20,6 +20,7 @@ def _wrapped_func(func_and_args):
         with _current.get_lock():
             _current.value += 1
         sys.stdout.write('\r\t{} of {}'.format(_current.value, _total.value))
+        sys.stdout.flush()
 
     return func(argument)