[llvm] [LangRef] Clarify that the pointer after an objet must be valid. (PR #127892)

[Nuno Lopes via llvm-commits]

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@@ -11722,8 +11722,9 @@ As a corollary, the only pointer in bounds of the null pointer in the default
 address space is the null pointer itself.
 
 These rules are based on the assumption that no allocated object may cross
-the unsigned address space boundary, and no allocated object may be larger
-than half the pointer index type space.
+the unsigned address space boundary, the pointer after the object must be valid,
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nunoplopes wrote:

I would express it in terms of overflow because null can have a different connotation in other address spaces.
I suggest a remark like:
> based on the assumption that no allocated object may cross the unsigned address space boundary (including a pointer after the end of the object).


https://github.com/llvm/llvm-project/pull/127892