[mlir][affinexpr] add parseAffineExpr to parser API

Similar to AffineMap and IntegerSet parsing, this change makes the more fine-grained AffineExpr available for general parsing, using a preset symbol set to recognize variables.

Motivation:
The AffineExpr parser will be used by the new sparse tensor encoding surface syntax. Originally, we planned to duplicate the affine parser completely, but that would be a terrible waste of a good thing. With this minor API change, we prepare the way for the sparse tensor dialect (and others) to reuse the AffineExpr parser outside the context of a more restricted AffineMap parser.

Reviewed By: Peiming

Differential Revision: https://reviews.llvm.org/D154177