[driver] handle removal of symlinks in infer-out

Summary:
This would fail before and works as expected now:

```
$ infer -- clang -c hello.c
$ cd infer-out/ && ln -s ../foo && cd ..
$ infer -- clang -c hello.c # crashes because it fails to delete infer-out/foo
```

Reviewed By: jberdine

Differential Revision: D4398763

fbshipit-source-id: 38465f8

infer/src/backend/infer.ml

@@ -18,8 +18,9 @@ module F = Format
 
 
 let rec rmtree name =
-  match Unix.opendir name with
+  match Unix.((lstat name).st_kind) with
-  | dir -> (
+  | S_DIR ->
+      let dir = Unix.opendir name in
       let rec rmdir dir =
         match Unix.readdir dir with
         | entry ->
@@ -31,8 +32,7 @@ let rec rmtree name =
             Unix.closedir dir ;
             Unix.rmdir name in
       rmdir dir
-    )
+  | _ ->
-  | exception Unix.Unix_error (Unix.ENOTDIR, _, _) ->
       Unix.unlink name
   | exception Unix.Unix_error (Unix.ENOENT, _, _) ->
       ()