Revert "Revert "YAMLIO: Encode ambiguous hex strings explicitly""

Don't quote octal compatible strings if they are only two wide, they
aren't ambiguous.

This reverts commit r205857 which reverted r205857.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205914 91177308-0d34-0410-b5e6-96231b3b80d8