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

[Hans Wennborg via cfe-commits]

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@@ -310,6 +310,10 @@ class DiagnosticsEngine : public RefCountedBase<DiagnosticsEngine> {
   // Suppress all diagnostics.
   bool SuppressAllDiagnostics = false;
 
+  // Force all system warnings to be shown, regardless of the current
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zmodem wrote:

(nit: the "all" feels redundant)

And maybe mention below that this does not get stored as location-specific state in modules.

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