YAMLIO: Fix string quoting logic.

YAMLIO printed a string as is without quotes unless it contains a newline
character. That did not suffice. We also need to quote a string if it starts
with a backquote, quote, double quote or atsign, or it's the empty string.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190469 91177308-0d34-0410-b5e6-96231b3b80d8