[MLIR][Parser] Fix AffineParser colliding bare identifiers with primitive types

The parser currently can't parse bare identifiers like 'i0' in affine
maps and sets, and similarly ids like f16/f32. But these bare ids are
part of the grammar - although they are primitive types.

```
error: expected bare identifier
set = affine_set<(i0, i1) : ()>
                   ^
```

This patch allows the parser for AffineMap/IntegerSet to parse bare
identifiers as defined by the grammer.

Reviewed By: bondhugula, rriddle

Differential Revision: https://reviews.llvm.org/D127076