[DebugInfo] Add expression decoding for `DW_OP_implicit_pointer` (#102923)
This allows `llvm-dwarfdump` to decode the DWARF 5 opcode
`DW_OP_implicit_pointer` (0xa0). GCC makes use of this opcode in recent
versions. LLVM contains some (unfinished) support as well. With existing
usage in the ecosystem, adding decoding support here seems reasonable.