[clang] [llvm] [clang][LLVM Demangler] Add a diagnostic that validates that all mang… (PR #111391)

[Viktoriia Bakalova via cfe-commits]

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@@ -41,7 +41,9 @@
 #include "clang/Basic/CharInfo.h"
 #include "clang/Basic/CodeGenOptions.h"
 #include "clang/Basic/Diagnostic.h"
+#include "clang/Basic/DiagnosticFrontend.h"
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VitaNuo wrote:

Sorry, most of those are a result of multiple iterations of this patch. 
The only necessary one is `llvm/Demangle/Demangle.h`.
Is that all right to add one include or should I still extract the code? 
Removing all the other includes for now.

https://github.com/llvm/llvm-project/pull/111391