[YAML] Add support for non-printable characters

[YAML] Add support for non-printable characters

LLVM IR function names which disable mangling start with '\01'
(https://www.llvm.org/docs/LangRef.html#identifiers).

When an identifier like "\01@abc@" gets dumped to MIR, it is quoted, but
only with single quotes.

http://www.yaml.org/spec/1.2/spec.html#id2770814:

"The allowed character range explicitly excludes the C0 control block
allowed), the surrogate block #xD800-#xDFFF, #xFFFE, and #xFFFF."

http://www.yaml.org/spec/1.2/spec.html#id2776092:

"All non-printable characters must be escaped.
[...]
Note that escape sequences are only interpreted in double-quoted scalars."

This patch adds support for printing escaped non-printable characters
between double quotes if needed.

Should also fix PR31743.

Differential Revision: https://reviews.llvm.org/D41290

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@320996 91177308-0d34-0410-b5e6-96231b3b80d8