[clang] [Modules] Enrich diags for out of date module dependencies (PR #190203)

[Qiongsi Wu via cfe-commits]

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@@ -132,24 +133,31 @@ ModuleManager::AddModuleResult ModuleManager::addModule(
 
   std::optional<ModuleFileKey> FileKey = FileName.makeKey(FileMgr);
   if (!FileKey) {
-    ErrorStr = "module file not found";
-    return Missing;
+    Result.K = AddModuleResult::Missing;
+    return Result;
   }
 
-  // Check whether we already loaded this module, before
+  // Check whether we already loaded this module before.
+  // Note: `isModuleFileOutOfDate` and `checkSignature` are mutally exclusive in
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qiongsiwu wrote:

Nit: probably a typo? `mutally` -> `mutually`. 

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