[cmake] Allow std::filesystem tests in CrossWinToARMLinux.cmake

Now that https://reviews.llvm.org/D78200 has landed, there should be
no problem with those tests. If there is, it is OK to revert this.

clang/cmake/caches/CrossWinToARMLinux.cmake

@@ -89,9 +89,6 @@ set(LIBCXXABI_LINK_TESTS_WITH_SHARED_LIBCXX    OFF CACHE BOOL "")
 set(LIBCXX_LINK_TESTS_WITH_SHARED_LIBCXXABI    OFF CACHE BOOL "")
 set(LIBCXX_LINK_TESTS_WITH_SHARED_LIBCXX       OFF CACHE BOOL "")
 
-# FIXME: Remove this when https://reviews.llvm.org/D78200 is merged.
-set(LIBCXX_ENABLE_FILESYSTEM OFF CACHE BOOL "") 
-
 set(LIBCXX_USE_COMPILER_RT                  ON CACHE BOOL "")
 set(LIBCXX_TARGET_TRIPLE                    "${CMAKE_C_COMPILER_TARGET}" CACHE STRING "")
 set(LIBCXX_SYSROOT                          "${DEFAULT_SYSROOT}" CACHE STRING "")