[lldb] Add RegisterFlags class

This models the "flags" node from GDB's target XML:
https://sourceware.org/gdb/onlinedocs/gdb/Target-Description-Format.html

This node is used to describe the fields of registers like cpsr on AArch64.

RegisterFlags is a class that contains a list of register fields.
These fields will be extracted from the XML sent by the remote.

We assume that there is at least one field, that the fields are
sorted in descending order and do not overlap. That will be
enforced by the XML processor (the GDB client code in our case).

The fields may not cover the whole register. To account for this
RegisterFields will add anonymous padding fields so that
sizeof(all fields) == sizeof(register). This will save a lot
of hasssle later.

Reviewed By: jasonmolenda, JDevlieghere

Differential Revision: https://reviews.llvm.org/D145566