[YAMLTraits] Fix mapping <none> value that followed by comments.

When mapping an optional value, if the value is <none> and followed
by comments, there will be a parsing error. This patch helps fix this
issue.

e.g.,

When mapping the following YAML,

```
Sections:
  - Name:  blah
    Type:  SHT_foo
    Flags: [[FLAGS=<none>]] ## some comments.
```

the raw value of `ScalarNode` is "<none> " rather than "<none>". We need
to remove the spaces.

Differential Revision: https://reviews.llvm.org/D85180