Resolve manual_ok_err clippy lint

Summary:
```lang=text
warning: manual implementation of `ok`
   --> fbcode/buck2/app/buck2_error/src/error.rs:252:17
    |
252 | /                 if let Ok(typed) = Arc::downcast(v.clone()) {
253 | |                     Some(typed)
254 | |                 } else {
255 | |                     None
256 | |                 }
    | |_________________^ help: replace with: `Arc::downcast(v.clone()).ok()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_ok_err
    = note: `-W clippy::manual-ok-err` implied by `-W clippy::all`
    = help: to override `-W clippy::all` add `#[allow(clippy::manual_ok_err)]`

warning: manual implementation of `ok`
   --> fbcode/buck2/app/buck2_events/src/dispatch.rs:394:5
    |
394 | /     match EVENTS.try_with(|dispatcher| dispatcher.dupe()) {
395 | |         Ok(dispatcher) => Some(dispatcher),
396 | |         Err(..) => None,
397 | |     }
    | |_____^ help: replace with: `EVENTS.try_with(|dispatcher| dispatcher.dupe()).ok()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_ok_err
    = note: `-W clippy::manual-ok-err` implied by `-W clippy::all`
    = help: to override `-W clippy::all` add `#[allow(clippy::manual_ok_err)]`

warning: manual implementation of `ok`
   --> fbcode/buck2/app/buck2_server/src/daemon/disk_state.rs:115:30
    |
115 |       let materializer_state = match load_result {
    |  ______________________________^
116 | |         Ok(s) => Some(s),
...   |
120 | |         Err(_e) => None,
121 | |     };
    | |_____^ help: replace with: `load_result.ok()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_ok_err
    = note: `-W clippy::manual-ok-err` implied by `-W clippy::all`
    = help: to override `-W clippy::all` add `#[allow(clippy::manual_ok_err)]`

warning: manual implementation of `ok`
   --> fbcode/buck2/app/buck2_server_commands/src/commands/build.rs:261:69
    |
261 |           let mut outputs = v.outputs.into_iter().filter_map(|output| match output {
    |  _____________________________________________________________________^
262 | |             Ok(output) => Some(output),
263 | |             _ => None,
264 | |         });
    | |_________^ help: replace with: `output.ok()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_ok_err
    = note: `-W clippy::manual-ok-err` implied by `-W clippy::all`
    = help: to override `-W clippy::all` add `#[allow(clippy::manual_ok_err)]`

warning: manual implementation of `ok`
   --> fbcode/buck2/starlark-rust/starlark/src/values/owned_frozen_ref.rs:113:9
    |
113 | /         match self.try_map_result(|x| f(x).ok_or(())) {
114 | |             Ok(x) => Some(x),
115 | |             Err(()) => None,
116 | |         }
    | |_________^ help: replace with: `self.try_map_result(|x| f(x).ok_or(())).ok()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_ok_err
    = note: `-W clippy::manual-ok-err` implied by `-W clippy::all`
    = help: to override `-W clippy::all` add `#[allow(clippy::manual_ok_err)]`

errors: 0; warnings: 5
```

Reviewed By: diliop

Differential Revision: D72737416

fbshipit-source-id: 481593ab24c4af5e59019ae6ebf84802ee5c05fd

Author: David Tolnay

starlark/src/values/owned_frozen_ref.rs

@@ -110,10 +110,7 @@ impl<'f, T: ?Sized> OwnedRefFrozenRef<'f, T> {
     where
         F: FnOnce(&'f T) -> Option<&'f U>,
     {
-        match self.try_map_result(|x| f(x).ok_or(())) {
-            Ok(x) => Some(x),
-            Err(()) => None,
-        }
+        self.try_map_result(|x| f(x).ok_or(())).ok()
     }
 }