[clang] [clang][DependencyScanning] Logging Dependency Scanning Events (PR #195896)

[Qiongsi Wu via cfe-commits]

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@@ -71,14 +75,19 @@ bool InMemoryModuleCache::tryToDropPCM(llvm::StringRef Filename) {
   auto &PCM = I->second;
   assert(PCM.Buffer && "PCM to remove is scheduled to be built...");
 
-  if (PCM.IsFinal)
+  if (PCM.IsFinal) {
+    Logger.log() << "pcm_not_dropped: " << Filename;
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qiongsiwu wrote:

The term comes from the enclosing method `nMemoryModuleCache::tryToDropPCM`. As far as I can tell this is only called in `ASTReader::ReadASTCore` to handle failure scenarios when `ShouldFinalizePCM` turns out to be false at exit. The intention is that we should log when we exit `ReadASTCore`, and indicate if the read is successful  (the case we call `MC.finalizePCM(FileName)`), or if it is not (the case we call `MC.tryToDropPCM(FileName)`). I don't think a lit test against this failure case is very easy to add, but I can add a unit test (adding some files but do not finalize them in the module cache to artificially trigger the failure in the unit test) if that is desirable. 

https://github.com/llvm/llvm-project/pull/195896