Add writeAsHex(raw_ostream &) method to BinaryRef.

This hides the implementation. A future commit will remove the
error-prone getHex() and getBinary() methods.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183352 91177308-0d34-0410-b5e6-96231b3b80d8