[yaml2obj] - Add a support for "<none>" value for all optional fields.

It implements an approach suggested in the D84398 thread.

With it the following:

```
Sections:
  - Name:   .bar
    Type:   SHT_PROGBITS
    Offset: [[MACRO=<none>]]
```

works just like the `Offset` key was not specified.
It is useful for tests that want to have a default value for a field and to
have a way to override it at the same time.

Differential revision: https://reviews.llvm.org/D84526