For example, the following declarations have the same USR with a single ERROR_TYPE parameter despite being distinct declarations.
```swift
func bar(body: Invalid) {}
func bar(ignoreCase: Bool, body: Invalid) {}
```
We originally intended to check the USR so that local rename behaves more like global rename, which also looks symbols up by USR. But the above example proves that assumption wrong.
rdar://126803702
This has been an unnecessary code path for a long time now and should be removed particularly because it triggers wasteful `stat` calls.
rdar://51523161
Having a single interface for source files and modules and having to
pass in a buffer id was a crappy API. This splits the interface to take
either a Module or a SourceFile and handles the buffer id stuff
internally.
