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

[akyrtzi]

Summary

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.

[benlangmuir]

I think we should verify the performance impact to running tests is minimal before landing this, like they did with the _GLIBCXX_ASSERTIONS change. Also CCing @ldionne in case he has opinions on us using libc++ assertions. Otherwise LGTM.

[akyrtzi]

In D143612#4114178, @benlangmuir wrote:

I think we should verify the performance impact to running tests is minimal before landing this, like they did with the _GLIBCXX_ASSERTIONS change.

It isn't scientific but I did one timing of running the llvm & clang tests before this change and one timing running them after, and after took a bit less time, so probably this is lost in the noise.

[ldionne]

I think this is fine.

[benlangmuir]

Since perf impact is minimal, LGTM.

llvm/cmake/modules/HandleLLVMOptions.cmake

if (MSVC)
           CMAKE_C_FLAGS_MINSIZEREL)
         string (REGEX REPLACE "(^| )[/-]D *NDEBUG($| )" " "
           "${flags_var_to_scrub}" "${${flags_var_to_scrub}}")
       endforeach()
     endif()
   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)
   add_compile_definitions(EXPENSIVE_CHECKS)
 
   # In some libstdc++ versions, std::min_element is not constexpr when
   # _GLIBCXX_DEBUG is enabled.
   CHECK_CXX_SOURCE_COMPILES("