[clang] [Clang][Modules] Ensure global diagnostic overrides are respected in system modules (PR #180684)

[Hans Wennborg via cfe-commits]

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@@ -549,8 +549,14 @@ DiagnosticIDs::getDiagnosticSeverity(unsigned DiagID, SourceLocation Loc,
   // If we are in a system header, we ignore it. We look at the diagnostic class
   // because we also want to ignore extensions and warnings in -Werror and
   // -pedantic-errors modes, which *map* warnings/extensions to errors.
-  if (State->SuppressSystemWarnings && Loc.isValid() &&
-      SM.isInSystemHeader(SM.getExpansionLoc(Loc))) {
+  //
+  // We check both the location-specific state and the global engine state.
+  // In some cases (like template instantiations from system modules), the
+  // location-specific state might have suppression enabled, but the global
+  // engine state might have an override (e.g. AllowWarningInSystemHeaders)
+  // to show the warning.
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zmodem wrote:

I think this comment should be updated to not talk about the global state, but the override specifically.

https://github.com/llvm/llvm-project/pull/180684