Avoid spurious breakpoint-setting failure in DAP

Avoid spurious breakpoint-setting failure in DAP

A user pointed out that if a DAP setBreakpoints request has a 'source'
field in a SourceBreakpoint object, then the gdb DAP implementation
will throw an exception.

While SourceBreakpoint does not allow 'source' in the spec, it seems
better to me to accept it.  I don't think we should fully go down the
"Postel's Law" path -- after all, we have the type-checker -- but at
the same time, if we do send errors, they should be intentional and
not artifacts of the implementation.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30820