[OpenMP][Offload] Support PRIVATE | ATTACH maps for corresponding-pointer-initialization. (llvm#160760)

`PRIVATE | ATTACH` maps can be used to represent firstprivate pointers
that should be initialized by doing doing the pointee's device address,
if its lookup succeeds, or retain the original host pointee's address
otherwise.

With this, for a test like the following:

  ```f90
  integer, pointer :: p(:)
  !$omp target map(p(1))
  ... print*, p(1)
  !$omp end target
  ```

The codegen can look like:
  ```llvm
   ; maps for p:
   ; &p(1),       &p(1), sizeof(p(1)),       TO|FROM              //(1)
   ; &ref_ptr(p), &p(1), sizeof(ref_ptr(p)), ATTACH               //(2)
   ; &ref_ptr(p), &p(1), sizeof(ref_ptr(p)), PRIVATE|ATTACH|PARAM //(3)
   call... @__omp_outlined...(ptr %ref_ptr_of_p)
  ```

* `(1)` maps the pointee `p(1)`.
* `(2)` attaches it to the (previously) mapped `ref_ptr(p)`, if present.
  It can be controlled via OpenMP 6.1's `attach(auto/always/never)`
  map-type modifiers.
* `(3)` privatizes and initializes the local `ref_ptr(p)`, which gets
passed
  in as the kernel argument `%ref_ptr_of_p`. Can be skipped if p is not
  referenced directly within the region.

While similar mapping can be used for C/C++, it's more important/useful
for Fortran as we can avoid creating another argument for passing the
descriptor, and use that to initialize the private copy in the body of
the kernel.

Author: abhinavgaba