[llvm] Allow empty range attribute and add assert for full range (PR #100601)

[Nikita Popov via llvm-commits]

================
@@ -1675,7 +1675,8 @@ Currently, only the following parameter attributes are defined:
     -  The pair ``a,b`` represents the range ``[a,b)``.
     -  Both ``a`` and ``b`` are constants.
     -  The range is allowed to wrap.
-    -  The range should not represent the full or empty set. That is, ``a!=b``.
+    -  Only for the empty set is ``a`` and ``b`` allowed to be equal and
+       only for the value 0.
----------------
nikic wrote:

I found this a bit hard to understand, maybe like this?
```suggestion
    -  The empty range is represented using ``0,0``.
    -  Otherwise, ``a`` and ``b` are not allowed to be equal.
```

https://github.com/llvm/llvm-project/pull/100601