[mlir] Added oilist primitive

This patch attempts to add the `oilist` primitive proposed in the [[ https://llvm.discourse.group/t/rfc-extending-declarative-assembly-format-to-support-order-independent-variadic-segments/4388 | RFC: Extending Declarative Assembly Format to support order-independent variadic segments ]].

This element supports optional order-independent variadic segments for operations. This will allow OpenACC and OpenMP Dialects to have similar and relaxed requirements while encouraging the use of Declarative Assembly Format and avoiding code duplication.

An oilist element parses grammar of the form:
```
clause-list := clause clause-list | empty
clause := `keyword` clause1 | `otherKeyword` clause2
clause1 := <assembly-format element>
clause2 := <assembly-format element>
```

AssemblyFormat specification:
```
let assemblyFormat = [{
  oilist( `keyword` clause1
        | `otherkeyword` clause2
        ...
        )
}];
```

Example:
```
oilist( `private` `(` $arg0 `:` type($arg0) `)`
      | `nowait`
      | `reduction` custom<ReductionClause>($arg1, type($arg1)))

oilist( `private` `=` $arg0 `:` type($arg0)
      | `reduction` `=` $arg1 `:` type($arg1)
      | `firstprivate` `=` $arg3 `:` type($arg2))
```

Reviewed By: Mogball, rriddle

Differential Revision: https://reviews.llvm.org/D115215