[mlir][flang][OpenMP] Privatize LINEAR list items with linear privatizers; support ALLOCATABLE (#228827)

Flang miscompiled `LINEAR` on an `ALLOCATABLE` list item (#182401). The
linear
operand was the descriptor reference, but `linear_var_types` held the
element
type, so translation created an undersized private copy and the program
crashed.

This patch makes linear list items use privatizer recipes, the same way
`private` and `reduction` do.

**MLIR**
- The `linear` clause on `omp.wsloop` and `omp.simd` now carries a
privatizer
  symbol and an entry block argument for each list item:
  ```mlir
  omp.private {type = linear} @x.linear : i32
omp.simd linear(@x.linear %x -> %arg0 : !llvm.ptr = %step : i32)
linear_var_types([i32]) {
  ```
- New `linear` `DataSharingType` for `omp.private`. Also an optional
  `data_address` region that yields the address of the underlying linear
  value when the privatized type wraps it (e.g. a Fortran descriptor).
- Verifier checks:
- a linear clause must use a `linear` privatizer whose type matches its
    list item;
  - `linear` privatizers can't be used in other clauses;
  - `data_address` is only allowed on `linear` privatizers, which can't
    have a copy region;
  - `omp.declare_simd` doesn't accept linear privatizers.

**Translation**
- Linear variables are allocated, initialized and deallocated through
the
  existing private-variable path.
- The start value is read from the original variable, the private copy
is
  set to `start + iv * step`, and the value from the last iteration is
copied back. All accesses go through the `data_address` region when the
  privatizer has one.
- The old `.linear_result` temporary and the in-place rewrite of the
  original variable are removed.

**Flang**
- `processLinear` and the implicit linear IV of `simd` now create
`linear`
  privatizers through `DataSharingProcessor`. For an allocatable, the
  recipe has a `data_address` region (`fir.load` + `fir.box_addr`).
- For composite `do simd` and `distribute parallel do simd`, linear
  privatizers go on `omp.wsloop`.
- Lowering linear without delayed privatization now emits a TODO.

Fixes #182401

GitOrigin-RevId: ac5d3955cd1931f71109caa8c026319ca69ca252