This represents the enum type that can be assigned to a field using the
`<enum>` element in the target XML.
https://sourceware.org/gdb/current/onlinedocs/gdb.html/Enum-Target-Types.html
Each enumerator has:
* A non-empty name
* A value that is within the range of the field it's applied to
The XML includes a "size" but we don't need that for anything and it's a
pain to verify so I've left it out of our internal structures. When
emitting XML we'll set size to the size of the register using the enum.
An Enumerator class is added to RegisterFlags and hooked up to the
existing ToXML so lldb-server can use it to emit enums as well.
As enums are elements on the same level as flags, when emitting XML
we'll do so via the registers. Before emitting a flags element we look
at all the fields and see what enums they reference. Then print all of
those if we haven't already done so.
Functions are added to dump enum information for `register info` to use
to show the enum information.
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