Convert MemRefCastOp to the LLVM IR dialect

Add support for converting `memref_cast` operations into the LLVM IR dialect.
This goes beyond want is currently implemented in the MLIR standard ops to LLVM
IR translation, but follows the general principles of the memref descriptors.
A memref cast creates a new descriptor containing the same buffer pointer but a
potentially different number of dynamic sizes (as many as dynamic dimensions in
the target memref type).  The lowering copies the buffer pointer to the new
descriptor and inserts dynamic sizes to it.  If the size is static in the
source type, a constant value is inserted as the dynamic size, otherwise a
dynamic value is copied from the source descriptor, taking into account the
difference in dynamic size positions in the descriptor.

PiperOrigin-RevId: 233082035