This teaches lldb to parse the enum XML elements sent by lldb-server,
and make use of the information in `register read` and `register info`.
The format is described in
https://sourceware.org/gdb/current/onlinedocs/gdb.html/Enum-Target-Types.html.
The target XML parser will drop any invalid enum or evalue. If we find
multiple evalue for the same value, we will use the last one we find.
The order of evalues from the XML is preserved as there may be good
reason they are not in numerical order.
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