[yaml2obj] - Add a way to exclude specified sections from the section header.

[yaml2obj] - Add a way to exclude specified sections from the section header.

This implements a new "Excluded" key that can be used
to exclude entries from section header:

```
SectionHeaderTable:
  Sections:
    ...
  Excluded:
    - Name: .foo
```

Differential revision: https://reviews.llvm.org/D81005