[cmake] Use LLVM_ENABLE_ASSERTIONS to enable assertions in libstdc++

On my machine this showed no new failures in check-llvm and increased
the testing time from 138 to 142 seconds, for a Release build with
assertions enabled.

Differential Revision: https://reviews.llvm.org/D142279

(cherry picked from commit b4d51ea)

Author: jayfoad

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -83,6 +83,8 @@ if( LLVM_ENABLE_ASSERTIONS )
       endforeach()
      endif()
   endif()
+  # Enable assertions in libstdc++.
+  add_compile_definitions(_GLIBCXX_ASSERTIONS)
 endif()
 
 if(LLVM_ENABLE_EXPENSIVE_CHECKS)