Include `count` in AppleClang toolchains.

The motivation here is so we can run the compiler-rt tests
from a standalone build against AppleClang.

In particular the `Posix/halt_on_error-torture.cpp` and
`Posix/halt_on_error_suppress_equal_pcs.cpp` ASan test cases currently
require this binary for the tests to pass.

rdar://76366784

Differential Revision: https://reviews.llvm.org/D100087
diff --git a/clang/cmake/caches/Apple-stage2.cmake b/clang/cmake/caches/Apple-stage2.cmake
index 3a8bae0..d347ffc 100644
--- a/clang/cmake/caches/Apple-stage2.cmake
+++ b/clang/cmake/caches/Apple-stage2.cmake
@@ -65,6 +65,7 @@
   FileCheck
   yaml2obj
   not
+  count
   CACHE STRING "")
 
 set(LLVM_DISTRIBUTION_COMPONENTS