[clang] [NFC] clang add DiagnosticsTestHelper decl (PR #112820)

[Jinsong Ji via cfe-commits]

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@@ -1024,6 +1024,9 @@ class DiagnosticsEngine : public RefCountedBase<DiagnosticsEngine> {
   /// @}
 };
 
+/// Declare the friend function for testing to avoid GCC warning.
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jsji wrote:

Sure.

https://github.com/llvm/llvm-project/pull/112820