[DWARFYAML][debug_info] Use 'AbbrCode' to index the abbreviation.

Before this patch, we use `(uint32_t)AbbrCode - (uint32_t)FirstAbbrCode` to index the abbreviation. It's impossible for we to use the preceeding abbreviation of the previous one (e.g., if the previous DIE's `AbbrCode` is 2, we are unable to use the abbreviation with index 1). In this patch, we use `AbbrCode` to index the abbreviation directly.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D82173