[CMake] Use LLVM_ENABLE_ASSERTIONS to enable the hardened mode in libc++.

Use the new libc++ hardened mode instead of the deprecated safe mode.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D156377

(cherry picked from commit 194e2ba)

Author: var-const

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -85,8 +85,8 @@ if( LLVM_ENABLE_ASSERTIONS )
   endif()
   # Enable assertions in libstdc++.
   add_compile_definitions(_GLIBCXX_ASSERTIONS)
-  # Enable assertions in libc++.
-  add_compile_definitions(_LIBCPP_ENABLE_ASSERTIONS)
+  # Enable the hardened mode in libc++.
+  add_compile_definitions(_LIBCPP_ENABLE_HARDENED_MODE)
 endif()
 
 if(LLVM_ENABLE_EXPENSIVE_CHECKS)