[llvm] [NFC] Move DroppedVariableStats code to Analysis (PR #120502)

[Mikhail Goncharov via llvm-commits]

metaflow wrote:

when https://github.com/llvm/llvm-project/commit/e389492d6a00e1c49a034e13343098541ebd03c6 landed `DroppedVariableStats.h` was 

```
...
#include "llvm/CodeGen/MachinePassManager.h"
#include "llvm/IR/DebugInfoMetadata.h"
#include "llvm/IR/DiagnosticInfo.
...
```

and depended on codegen. In https://github.com/llvm/llvm-project/commit/0b5b09b67c572867d88bbf5b41bcc5e722ec653a you removed this dependency.

https://github.com/llvm/llvm-project/pull/120502