[Remarks] Emit variable info in auto-init remarks

This enhances the auto-init remark with information about the variable
that is auto-initialized.

This is based of debug info if available, or alloca names (mostly for
development purposes).

```
auto-init.c:4:7: remark: Call to memset inserted by -ftrivial-auto-var-init. Memory operation size: 4096 bytes.Variables: var (4096 bytes). [-Rpass-missed=annotation-remarks]
  int var[1024];
      ^
```

This allows to see things like partial initialization of a variable that
the optimizer won't be able to completely remove.

Differential Revision: https://reviews.llvm.org/D97734