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

[Qiongsi Wu via cfe-commits]

qiongsiwu wrote:

> Is the intended usage to rely only on pre-defined events or to add custom logging too?

The events can be added quite easily within the Dependency Scanning stack. When an instance of the logger is available, to add a new log, one can do 
```
Logger.log() << "new even to log" << ... ;
```

So this can be used similarly to `llvm::errs()` if a Logger instance is in scope. 


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