[clang] [Modules] Enrich diags for out of date module dependencies (PR #190203)

[Qiongsi Wu via cfe-commits]

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@@ -120,6 +121,18 @@ class InputFile {
   bool isNotFound() const { return Val.getInt() == NotFound; }
 };
 
+/// Describes a single change detected in a module file or input file.
+struct Change {
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qiongsiwu wrote:

Nits: 

1. Do we want to default initialize Kind? 
2. I slightly prefer adding this struct definition publicly to `class AddModuleResult`. I think this definition is closely related to `AddModuleResult` and used with `AddModuleResult`, hence my preference. I also understand it is easier to do it here since we use this definition in `ASTReader.cpp` a lot so I don't feel strongly.

https://github.com/llvm/llvm-project/pull/190203