[YAML] Use correct source location for unknown key errors.

Currently unknown keys when inputting mapping traits have the location set to the Value.
Example:
```
YAML:1:14: error: unknown key 'UnknownKey'
{UnknownKey: SomeValue}
             ^~~~~~~~~
```
This is unhelpful for a user as it draws them to fix the wrong item.

Reviewed By: silvas

Differential Revision: https://reviews.llvm.org/D93037