Add writeAsBinary(raw_ostream &) method to BinaryRef.

Previously, yaml2coff.cpp had a writeHexData static helper function to
do this, but it is generally useful functionality.

Also, validate hex strings up-front to avoid running having to handle
errors "deep inside" the yaml2obj code (it also gives better diagnostics
than it used to).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183345 91177308-0d34-0410-b5e6-96231b3b80d8