Don't use std::__clz in 'test/support/hexfloat.h'.

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.

llvm-svn: 268354