Properly initialize CXX_SUPPORTS_COVERED_SWITCH_DEFAULT_FLAG CMake variable

A follow-up for #92953. Addresses the following buildbot failures:
https://lab.llvm.org/buildbot/#/builders/36/builds/45836
https://lab.llvm.org/buildbot/#/builders/57/builds/35200

Author: Endilll

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -817,7 +817,10 @@ if (LLVM_ENABLE_WARNINGS AND (LLVM_COMPILER_IS_GCC_COMPATIBLE OR CLANG_CL))
   endif()
 
   append("-Wimplicit-fallthrough" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
+
+  set(CXX_SUPPORTS_COVERED_SWITCH_DEFAULT_FLAG 0)
   if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+    set(CXX_SUPPORTS_COVERED_SWITCH_DEFAULT_FLAG 1)
     append("-Wcovered-switch-default" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
   endif()
   append_if(USE_NO_UNINITIALIZED "-Wno-uninitialized" CMAKE_CXX_FLAGS)