[mlir][bufferization] Privatize buffers for parallel regions

One-Shot Bufferize correctly handles RaW conflicts around repetitive regions (loops). Specical handling is needed for parallel regions. These are a special kind of repetitive regions that can have additional RaW conflicts that would not be present if the regions would be executed sequentially.

Example:
```
%0 = bufferization.alloc_tensor()
scf.forall ... {
  %1 = linalg.fill ins(...) outs(%0)
  ...
  scf.forall.in_parallel {
    tensor.parallel_insert_slice %1 into ...
  }
}
```

A separate (private) buffer must be allocated for each iteration of the `scf.forall` loop.

This change adds a new interface method to `BufferizableOpInterface` to detect parallel regions. By default, regions are assumed to be sequential.

A buffer is privatized if an OpOperand bufferizes to a memory read inside a parallel region that is different from the parallel region where operand's value is defined.

Differential Revision: https://reviews.llvm.org/D159286