[MLIR] Introduce op trait PolyhedralScope (revised)

(A previous version of this, dd2c639c, was
reverted.)

Introduce op trait PolyhedralScope for ops to define a new scope for
polyhedral optimization / affine dialect purposes, thus generalizing
such scopes beyond FuncOp. Ops to which this trait is attached will
define a new scope for the consideration of SSA values as valid symbols
for the purposes of polyhedral analysis and optimization. Update methods
that check for dim/symbol validity to work based on this trait.

Differential Revision: https://reviews.llvm.org/D79060