[clang] Clarify use of contractions in diagnostic messages (PR #116803)

[Erich Keane via cfe-commits]

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@@ -160,6 +160,10 @@ wording a diagnostic.
   named in a diagnostic message. e.g., prefer wording like ``'this' pointer
   cannot be null in well-defined C++ code`` over wording like ``this pointer
   cannot be null in well-defined C++ code``.
+* Prefer diagnostic wording without contractions whenever possible. The single
+  quote in a contraction can be visually distracting due to its use with
+  syntactic constructs and contractions can be harder to understand for non-
+  native English speakers.
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erichkeane wrote:

As a native speaker, (and looking in a dictionary), they are identical meaning (same as can't).  

We DID have plenty of `can not` in both comments and diagnostics, but they were recently changed.

https://github.com/llvm/llvm-project/pull/116803