[clang] [WIP] Implement `print-deserialized-declarations` flag to dump source… (PR #133910)

[Viktoriia Bakalova via cfe-commits]

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@@ -49,6 +51,135 @@ LLVM_INSTANTIATE_REGISTRY(FrontendPluginRegistry)
 
 namespace {
 
+/// Dumps deserialized declarations.
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VitaNuo wrote:

> If you want, you might use a comment to explain other non-trivial details, though, e.g. how the class must be used or why it implements certain interfaces and when the output happens.

This is a good idea. I have written an extensive comment, PTAL.

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