[lldb-dap] Fix source references (#144364)
The
[protocol](https://microsoft.github.io/debug-adapter-protocol//specification.html#Types_Source)
expects that `sourceReference` be less than `(2^31)-1`, but we currently
represent memory address as source reference, this can be truncated
either when sending through json or by the client. Instead, generate new
source references based on the memory address.
Make the `ResolveSource` function return an optional source.
