[CMake] Use LLVM_ENABLE_ASSERTIONS to enable assertions in libc++

Follow-up to https://reviews.llvm.org/D142279 for also enabling `libc++` assertions.
This is particularly useful to recover the assertion checks we had with `llvm::Optional`, before moving to `std::optional`.

Differential Revision: https://reviews.llvm.org/D143612

(cherry picked from commit d8e9a1a)

Author: akyrtzi

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -85,6 +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)
 endif()
 
 if(LLVM_ENABLE_EXPENSIVE_CHECKS)