[mlir][openacc] Add private representation

Currently privatization is not well represented in the OpenACC dialect. This
patch is a prototype to model privatization with a dedicated operation that
declares how the private value is created and destroyed.

The newly introduced operation is `acc.private.recipe` and it declares
an OpenACC privatization. The operation requires one mandatory and
one optional region.

  1. The initializer region specifies how to create and initialize a new
     private value. For example in Fortran, a derived-type might have a
     default initialization. The region has an argument that contains the
     value that need to be privatized. This is useful if the type is not
     a known at compile time and the private value is needed to create its
     copy.
  2. The destroy region specifies how to destruct the value when it reaches
     its end of life. It takes the privatized value as argument.

A same privatization can be used for multiple operand if they have the same
type and do not require a specific default initialization.

Example:

```mlir
acc.private.recipe @privatization_f32 : f32 init {
^bb0(%0: f32):
  // init region contains a sequence of operations to create and initialize the
  // copy if needed.
} destroy {
^bb0(%0: f32):
  // destroy region contains a sequences of operations to destruct the created
  // copy.
}

// The privatization symbol is then used in the corresponding operation.
acc.parallel private(@privatization_f32 -> %a : f32) {
}
```

Reviewed By: razvanlupusoru, jeanPerier

Differential Revision: https://reviews.llvm.org/D150622