std::__clz is a libc++ specific function so it can't be used in the test suite.
This patch implements a dumb "count leading zeros" implementation within
hexfloat itself.
This patch also fixes UB since the output of `__builtin_clz(0)` is undefined
according to the GCC docs.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@268354 91177308-0d34-0410-b5e6-96231b3b80d8