Fix register-setting response from DAP

Fix register-setting response from DAP

Andry noticed that given a DAP setExpression request, where the
expression to set is a register, DAP will return the wrong value -- it
will return the old value, not the updated one.

This happens because gdb.Value.assign (which was recently added for
DAP) does not update the value.

In this patch, I chose to have the assign method update the Value
in-place.  It's also possible to have it return a new value, but this
didn't seem very useful to me.