[llvm] [ADT] Simplify a constructor of iterator_range (NFC) (PR #158005)

[Thomas Köppe via llvm-commits]

tkoeppe wrote:

Ah, I was just going to touch this code, but now I need to rethink how to do it: The constructor template is unfortunately insufficiently constrained and "hides" the special member, so a constraint should be inserted somewhere that `remove_cvref_t<Container>` is not the same as `iterator_range`. (That was straight-forward to do in the previous version of the the code.)

https://github.com/llvm/llvm-project/pull/158005