ADT: Add sentinel tracking and custom tags to ilists

This adds two declarative configuration options for intrusive lists
(available for simple_ilist, iplist, and ilist).  Both of these options
affect ilist_node interoperability and need to be passed both to the
node and the list.  Instead of adding a new traits class, they're
specified as optional template parameters (in any order).

The two options:

 1. Pass ilist_sentinel_tracking<true> or ilist_sentinel_tracking<false>
    to control whether there's a bit on ilist_node "prev" pointer
    indicating whether it's the sentinel.  The default behaviour is to
    use a bit if and only if LLVM_ENABLE_ABI_BREAKING_CHECKS.

 2. Pass ilist_tag<TagA> and ilist_tag<TagB> to allow insertion of a
    single node into two different lists (simultaneously).

I have an immediate use-case for (1) ilist_sentinel_tracking: fixing the
validation semantics of MachineBasicBlock::reverse_iterator to match
ilist::reverse_iterator (ala r280032: see the comments at the end of the
commit message there).  I'm adding (2) ilist_tag in the same commit to
validate that the options framework supports expansion.  Justin Bogner
mentioned this might enable a possible cleanup in SelectionDAG, but I'll
leave this to others to explore.  In the meantime, the unit tests and
the comments for simple_ilist and ilist_node have usage examples.

Note that there's a layer of indirection to support optional,
out-of-order, template paramaters.  Internal classes are templated on an
instantiation of the non-variadic ilist_detail::node_options.
User-facing classes use ilist_detail::compute_node_options to compute
the correct instantiation of ilist_detail::node_options.

The comments for ilist_detail::is_valid_option describe how to add new
options (e.g., ilist_packed_int<int NumBits>).

llvm-svn: 281167

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