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

[Shafik Yaghmour via cfe-commits]

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@@ -2761,6 +2761,20 @@ are listed below.
    If ``-falign-functions`` is specified, or if the function has an
    ``aligned`` attribute, this option is ignored.
 
+.. option:: -fstable-pointer-subtraction
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shafik wrote:

The command line reference tends to be pretty sparse and I we have gotten a lot of feedback we could do better at documenting and I think this level of detail is helpful.

Maybe we could have it organized better though. 

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