debugger: improve robustness of watchers

This solves one common case of adding the 'class' *(class X*)0xdeadbeef
"needed" by gdb in case a local variable at the same location is known.

Change-Id: I5b25530b00e512c6e9155fb111ff4dc916832074
Reviewed-on: http://codereview.qt.nokia.com/76
Reviewed-by: hjk