Use ATTACH maps for array-sections/subscripts on pointers.

[abhinavgaba]

This is the initial clang change to support using ATTACH map-type for pointer-attachment.

This builds upon the following:

• [Offload] Introduce ATTACH map-type support for pointer attachment. llvm/llvm-project#149036
• [Clang][OpenMP] Capture mapped pointers on target by reference. llvm/llvm-project#145454

For example, for the following:

  int *p;
  #pragma omp target enter data map(p[1:10])

The following maps are now emitted by clang:

  (A)
  &p[0], &p[1], 10 * sizeof(p[1]), TO | FROM
  &p, &p[1], sizeof(p), ATTACH

Previously, the two possible maps emitted by clang were:

  (B)
  &p[0], &p[1], 10 * sizeof(p[1]), TO | FROM

  (C)
  &p, &p[1], 10 * sizeof(p[1]), TO | FROM | PTR_AND_OBJ

(B) does not perform any pointer attachment, while (C) also maps the
pointer p, both of which are incorrect.

---

With this change, we are using ATTACH-style maps, like (A), for cases where the expression has a base-pointer. For example:

  int *p, **pp;
  S *ps, **pps;
  ... map(p[0])
  ... map(p[10:20])
  ... map(*p)
  ... map(([20])p)
  ... map(ps->a)
  ... map(pps->p->a)
  ... map(pp[0][0])
  ... map(*(pp + 10)[0])

We also group mapping of clauses with the same base decl in the order of the increasing complexity of their base-pointers, e.g. for something like:

  S **spp;
  map(spp[0][0], spp[0][0].a), // attach-ptr: spp[0]
  map(spp[0]),                // attach-ptr: spp
  map(spp),                   // attach-ptr: N/A

We first map spp, then spp[0] then spp[0][0] and spp[0][0].a.

This allows us to also group "struct" allocation based on their attach pointers.

Cases that need handling:

• When a class member like p is a base-pointer in a map from a member function within the same class, p is not being privatized, instead, we still try to create an implicit map of this[0:1], and access p through that, which is incorrect.

 struct S { int *p;
 void f1() {
   #pragma omp target data map(p[0:1])
      printf("%p %p\n", &p, p);
 }

• Attach-style maps for declare mappers. That should be a separate PR.
• use_device_addr clause does not work properly, because we don't have a proper component-list set-up for it, just one component, so we cannot find the proper attach-ptr. For use_device_addr, we should match existing maps whose attach-ptr matches the attach-ptr of the use_device_addr operand.
• use_device_ptr handling has some issues too. Need debugging.
• Other issues that haven't been found yet.

Some tests still haven't been updated. These include:

  Clang :: OpenMP/copy-gaps-1.cpp
  Clang :: OpenMP/copy-gaps-6.cpp
  Clang :: OpenMP/map_struct_ordering.cpp
  Clang :: OpenMP/target_data_use_device_addr_codegen.cpp
  Clang :: OpenMP/target_data_use_device_ptr_codegen.cpp
  Clang :: OpenMP/target_enter_data_codegen.cpp
  Clang :: OpenMP/target_enter_data_depend_codegen.cpp
  Clang :: OpenMP/target_exit_data_codegen.cpp
  Clang :: OpenMP/target_exit_data_depend_codegen.cpp
  Clang :: OpenMP/target_map_codegen_18c.cpp
  Clang :: OpenMP/target_map_codegen_18d.cpp
  Clang :: OpenMP/target_map_codegen_28.cpp
  Clang :: OpenMP/target_map_codegen_29.cpp
  Clang :: OpenMP/target_map_codegen_31.cpp
  Clang :: OpenMP/target_map_codegen_hold.cpp
  Clang :: OpenMP/target_map_deref_array_codegen.cpp
  Clang :: OpenMP/target_map_member_expr_codegen.cpp
  Clang :: OpenMP/target_update_codegen.cpp
  Clang :: OpenMP/target_update_depend_codegen.cpp