YAMLIO: Encode ambiguous hex strings explicitly

YAMLIO would turn a BinaryRef into the string 0000000004000000.
However, the leading zero causes parsers to interpret it as being an
octal number instead of a hexadecimal one.

Instead, escape such strings as needed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205839 91177308-0d34-0410-b5e6-96231b3b80d8