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.
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