[mlir][affineexpr] Changing `AsmParser::parseAffineExpr` API to use ArrayRef

The methods added by D154177 don't require the `symbolSet` parameter to be mutable nor to have the `SmallVectorImpl` type, so this commit changes them to accept `ArrayRef` instead: both for generality, and to make the non-mutation an explicit part of the API.

Reviewed By: aartbik, Peiming

Differential Revision: https://reviews.llvm.org/D154751