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

[Erich Keane 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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erichkeane wrote:

couldn't we just put this at the top of the unittest file where this is defined/used?  Just do:

`namespace clang {
<declaration>
}`

It seems silly to do this for a function that is only used in the unittests.

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