[MLIR] Introduce an op trait that defines a new scope for auto allocation

Introduce a new operation property / trait (AutomaticAllocationScope)
for operations with regions that define a new scope for automatic allocations;
such allocations (typically realized on stack) are automatically freed when
control leaves such ops' regions. std.alloca's are freed at the closest
surrounding op that has this trait. All FunctionLike operations should normally
have this trait.

Differential Revision: https://reviews.llvm.org/D77787