[llvm] Add DISubrangeType (PR #126772)

[Tom Tromey via llvm-commits]

tromey wrote:

> Definitely a failure in this patch, but I don't really understand it. Will try to find out.

Valgrind helped track it down.  I was missing an initializer in `MDNodeKeyImpl<DISubrangeType>`.  I would have thought some warning would have triggered for this.  Anyway will upload the new patch once rebuilding + testing is done.

https://github.com/llvm/llvm-project/pull/126772