Fix clang tidy error and crash (#42564)

Clang tidy checks for #42399 are
failing for `formats_mtl.mm` and `context_mtl.mm`. Weirdly this happens
even though none of these files where modified in the branch.

For `context_mtl.mm` the fix is straightforward, just a missing
`std::move`.
For `formats_mtl.mm` clang-tidy actually crashes on the assignments to
`des.depthCompareFunction` and `des.depth_write_enabled`. The crash
doesn't happen if the assignment is done through a temporary variable.

I'm not sure if we should have a workaround for the crash or disable
clang-tidy on the file completely (FLUTTER_NOLINT).

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Author: knopp

impeller/renderer/backend/metal/context_mtl.mm

@@ -228,7 +228,7 @@ static bool DeviceSupportsComputeSubgroups(id<MTLDevice> device) {
   auto context = std::shared_ptr<ContextMTL>(new ContextMTL(
       device,
       MTLShaderLibraryFromFileData(device, shader_libraries_data, label),
-      worker_task_runner, std::move(is_gpu_disabled_sync_switch)));
+      std::move(worker_task_runner), std::move(is_gpu_disabled_sync_switch)));
   if (!context->IsValid()) {
     FML_LOG(ERROR) << "Could not create Metal context.";
     return nullptr;

impeller/renderer/backend/metal/formats_mtl.mm

@@ -64,8 +64,13 @@
 
   auto des = [[MTLDepthStencilDescriptor alloc] init];
 
-  des.depthCompareFunction = ToMTLCompareFunction(depth->depth_compare);
-  des.depthWriteEnabled = depth->depth_write_enabled;
+  // These temporary variables are necessary for clang-tidy (Fuchsia LLVM
+  // version 17.0.0git) to not crash.
+  auto compare_function = ToMTLCompareFunction(depth->depth_compare);
+  auto depth_write_enabled = depth->depth_write_enabled;
+
+  des.depthCompareFunction = compare_function;
+  des.depthWriteEnabled = depth_write_enabled;
 
   if (front.has_value()) {
     des.frontFaceStencil = ToMTLStencilDescriptor(front.value());