[broken rL203058] llvm/IR/Metadata.h NamedMDNode iterator broke Mesa with libcxx

Hi,

I've been unable to compile Mesa on my Gentoo box with LLVM and libcxx
for some time, and only now got the time to hunt down the bug:

TL;DR: rL203058 mentions your review which isn't linked in
Phabricator, and it makes NamedMDNode's iterator template<T1,T2> for a
reason unknown to me. This breaks Mesa which wants to use this
iterator to construct vector<const MDNode *> from {op_begin, op_end}.

If you believe this is Mesa's fault, I'd like to know why (and why
does LLVM bother exporting an std::iterator if people aren't supposed
to use it).

See the following URLs for the interaction:

https://cgit.freedesktop.org/mesa/mesa/tree/src/gallium/state_trackers/clover/llvm/metadata.hpp#n41
https://github.com/llvm-mirror/llvm/blob/master/include/llvm/IR/Metadata.h#L1293

Facts:
- C++11 has some special cases for std::vector<> with forward
iterators, which GCC doesn't implement. This makes Mesa's build
succeed against stdc++ (tested GCC 6.3.0).
- ยง23.2.3, table 100 wants vector<const MDNode*> to be
EmplaceConstructible from NamedMDNode::const_op_iterator::operator *,
which it is.
- However, libcxx's specialisation of this constructor wants
is_constructible<const MDNode *,
iterator_traits<const_op_iterator>::reference>. Due to your change in
rL203058, that reference is "T2 = MDNode" because the iterator
inherits from std::iterator<bidi_tag, T2>. const* type is not
constructible from the value type (I haven't seen in 14882:2011(E) why
it should), so the build fails.

What I want to know is why did you introduce T2 only to pass it into
the parent std::iterator<> class and not anywhere else. Changing the
inheritance to "T1 = const MDNode*" makes it all work again.

Thanks!