[clang] [clang][diagnostics] add '-Wundef-true' warning option (PR #128265)

[Richard Smith via cfe-commits]

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@@ -791,6 +791,8 @@ def ReservedIdAsMacroAlias : DiagGroup<"reserved-id-macro", [ReservedIdAsMacro]>
 def ReservedAttributeIdentifier : DiagGroup<"reserved-attribute-identifier">;
 def RestrictExpansionMacro : DiagGroup<"restrict-expansion">;
 def FinalMacro : DiagGroup<"final-macro">;
+def UndefinedIdentifier : DiagGroup<"undef">;
+def UndefinedTrueIdentifier : DiagGroup<"undef-true", [UndefinedIdentifier]>;
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zygoloid wrote:

This looks reversed to me: I think we want `UndefinedTrueIdentifier` to be listed as a child of `UndefinedIdentifier`, not the other way around, so that (eg) `-Wundef` implies `-Wundef-true` (not vice versa).

https://github.com/llvm/llvm-project/pull/128265