[yaml2obj] - Stop using square brackets for unique suffixes.

For describing section/symbol names we can use unique suffixes,
e.g:

```
- Name: '.foo [1]`
- Name: '.foo [2]`
```

It can be a problem (see https://reviews.llvm.org/D79984#inline-734829),
because `[]` are sometimes used to describe a macros:

```
- Name: "[[a0]]"
```

Seems the better approach is to use something else, like "()".
This patch does it and refactors the code related.

Differential revision: https://reviews.llvm.org/D80123