[clang] [clang] Add flag for relaxed pointer subtraction (PR #196392)

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@@ -113,6 +113,9 @@ latest release, please see the [Clang Web Site](https://clang.llvm.org) or the
 
 ### New Compiler Flags
 
+- New option `-fstable-pointer-subtraction` added to preserve stable semantics
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eleviant wrote:

I don't mind, but the main goal is to have option for defined pointer subtraction rather then rounding. Can we replace it with `-fdefined-pointer-subtraction` instead?

https://github.com/llvm/llvm-project/pull/196392