[clang] [Clang] Explain why a type trait evaluated to false. (PR #141238)

[Erich Keane via cfe-commits]

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@@ -1762,6 +1762,29 @@ def err_user_defined_msg_constexpr : Error<
   "%sub{subst_user_defined_msg}0 must be produced by a "
   "constant expression">;
 
+// Type traits explanations
+def note_unsatisfied_trait : Note<"%0 is not %enum_select<TraitName>{"
+                                  "%TriviallyRelocatable{trivially relocatable}"
+                                  "}1">;
+
+def note_unsatisfied_trait_reason
+    : Note<"because it "
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erichkeane wrote:

My word... I'm VERY glad we have `enum_select` :D 

https://github.com/llvm/llvm-project/pull/141238