When writing YAML in libclang, use yaml::escape instead of write_escaped

The YAMLParser has its own escaped string representation, and does not
handle octal escape sequences. When writing the virtual file system to a
YAML file, use yaml::escape().

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206443 91177308-0d34-0410-b5e6-96231b3b80d8