[YAML] Quote multiline string scalars

Summary:
Otherwise, the YAML parser breaks when trying to read them back in
'key: multiline_string_value' cases.

This patch fixes a problem when serializing structs which contain multi-line strings.
E.g., if we try to serialize  the following struct
```
{ "key1": "first line\nsecond line",
  "key2": "another string" }`
```

Before this patch, we got the YAML output that failed to parse:
```
key1: first line
second line
key2: another string
```

After the patch, we get:
```
key1: 'first line
second line'
key2: another string
```

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D47468

llvm-svn: 333527