[clang] [WIP] Implement `-dump-deserialized-declaration-ranges` flag. (PR #133910)

[Ilya Biryukov via cfe-commits]

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@@ -49,6 +51,150 @@ LLVM_INSTANTIATE_REGISTRY(FrontendPluginRegistry)
 
 namespace {
 
+/// DeserializedDeclsLineRangePrinter dumps ranges of deserialized declarations
+/// to aid debugging and bug minimization. It implements ASTConsumer and
+/// ASTDeserializationListener, so that an object of
+/// DeserializedDeclsLineRangePrinter registers as its own listener. The
+/// ASTDeserializationListener interface provides the DeclRead callback that we
+/// use to collect the deserialized Decls. Note that printing or otherwise
+/// processing them as this point is dangerous, since that could trigger
+/// additional deserialization and crash compilation. Therefore, we process the
+/// collected Decls in HandleTranslationUnit method of ASTConsumer. This is a
+/// safe point, since we know that by this point all the Decls needed by the
+/// compiler frontend have been deserialized. In case our processing causes
+/// further deserialization, DeclRead from the listener might be called again.
+/// However, at that point we don't accept any more Decls for processing.
+class DeserializedDeclsLineRangePrinter : public ASTDeserializationListener,
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ilya-biryukov wrote:

NIT: let's inherit form `ASTDeserializationListener` privately to reduce the exposed public interface.

https://github.com/llvm/llvm-project/pull/133910