[Flang] Add the FIR LLVMPointer Type

Add a fir.llvm_ptr type to allow any level of indirections

Currently, fir pointer types (fir.ref, fir.ptr, and fir.heap) carry
a special Fortran semantics, and cannot be freely combined/nested.

When implementing some features, lowering sometimes needs more liberty
regarding the number of indirection levels. Add a fir.llvm_ptr that has
no constraints.

Allow its usage in fir.coordinate_op, fir.load, and fir.store.

Convert the FIR LLVMPointer to an LLVMPointer in the LLVM dialect.

Reviewed By: clementval

Differential Revision: https://reviews.llvm.org/D113755



Co-authored-by: Jean Perier <jperier@nvidia.com>