[flang][acc] Implement PointerLikeType API gen[Allocate/Free/Copy] (#163660)

Implements genAllocate, genFree, and genCopy for FIR pointer types
(fir.ref, fir.ptr, fir.heap, fir.llvm_ptr) in the OpenACC
PointerLikeType interface.

- genAllocate: Uses fir.alloca for stack types, fir.allocmem for heap
types. Returns null for dynamic/unknown types (unlimited polymorphic,
dynamic arrays, dynamic character lengths, box types).
- genFree: Generates fir.freemem for heap allocations. Returns false if
original allocation cannot be found.
- genCopy: Uses fir.load+fir.store for trivial types (scalars),
hlfir.assign for non-trivial types (arrays, derived types, characters).
Returns false for unsupported dynamic types and box types.

Adds comprehensive MLIR tests covering various FIR types and edge cases.

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