Fix a nuisance compiler warning from clang (#12144)

Summary:
Example:

```
cache/clock_cache.cc:56:7: error: fallthrough annotation in unreachable code [-Werror,-Wimplicit-fallthrough]
      FALLTHROUGH_INTENDED;
      ^
./port/lang.h:10:30: note: expanded from macro 'FALLTHROUGH_INTENDED'
                             ^
```

In clang < 14, this is annoyingly generated from -Wimplicit-fallthrough, but was changed to -Wunreachable-code-fallthrough (implied by -Wunreachable-code) in clang 14. See https://reviews.llvm.org/D107933 for how this nuisance pattern generated false positives similar to ours in the Linux kernel.

Just to underscore the ridiculousness of this warning, here an error is reported on the annotation, not the call to do_something(), depending on the constexpr value (https://godbolt.org/z/EvxqdPTdr):

```
#include <atomic>
void do_something();
void test(int v) {
    switch (v) {
        case 1:
            if constexpr (std::atomic<long>::is_always_lock_free) {
                return;
            } else {
                do_something();
                [[fallthrough]];
            }
        case 2:
            return;
    }
}
```

Pull Request resolved: #12144

Test Plan: Added the warning to our Makefile for USE_CLANG, which reproduced the warning-as-error as shown above, but is now fixed.

Reviewed By: jaykorean

Differential Revision: D52139615

Pulled By: pdillinger

fbshipit-source-id: ba967ae700c0916d1a478bc465cf917633e337d9

Makefile

@@ -539,7 +539,7 @@ endif
 
 ifdef USE_CLANG
 	# Used by some teams in Facebook
-	WARNING_FLAGS += -Wshift-sign-overflow -Wambiguous-reversed-operator
+	WARNING_FLAGS += -Wshift-sign-overflow -Wambiguous-reversed-operator -Wimplicit-fallthrough
 endif
 
 ifeq ($(PLATFORM), OS_OPENBSD)

cache/clock_cache.cc

@@ -51,14 +51,15 @@ inline uint64_t GetInitialCountdown(Cache::Priority priority) {
   switch (priority) {
     case Cache::Priority::HIGH:
       return ClockHandle::kHighCountdown;
-    default:
-      assert(false);
-      FALLTHROUGH_INTENDED;
     case Cache::Priority::LOW:
       return ClockHandle::kLowCountdown;
     case Cache::Priority::BOTTOM:
       return ClockHandle::kBottomCountdown;
   }
+  // Switch should have been exhaustive.
+  assert(false);
+  // For release build, fall back on something reasonable.
+  return ClockHandle::kLowCountdown;
 }
 
 inline void MarkEmpty(ClockHandle& h) {