Fix the following error when NDEBUG is defined:

include/llvm/Support/UnicodeCharRanges.h:56:5: error:
use of this statement in a constexpr constructor is a C++1y extension
[-Werror,-Wc++1y-extensions]
     assert(rangesAreValid());
          ^

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189599 91177308-0d34-0410-b5e6-96231b3b80d8