Emit DW_RLE_base_addressx + DW_RLE_offset_pairs

instead of DW_ELE_start_length in debug_rnglists section

This patch tries to reduce the size of the debug_rnglist section by
replacing the DW_RLE_start_length opcodes currently emitted by dsymutil
in favor of using DW_RLE_base_addressx + DW_RLE_offset_pair instead.

The DW_RLE_start_length is one AddressSize followed by a ULEB per entry,
whereas, the DW_RLE_base_addressx + DW_RLE_offset_pair will use one ULEB
for the base address, and then the DW_RLE_offset_pair is a pair of
ULEBs. This will be more efficient.

Differential Revision: https://reviews.llvm.org/D156166