watch set expression's default type was wrong with new modify type

`watch set expression` was passing the OptionGroupWatchpoint enum
in to Target where the LLDB_WATCH_TYPE_* bitfield was expected.
Modify matched READ|WRITE and resulted in a test failure in
TestWatchTaggedAddress.py.  David temporarily changed the test to
expect this incorrect output; this fixes the bug and updates the
test case to test it for correctness again.