Introduce and use a new section type for the bb_addr_map section.

This patch lets the bb_addr_map (renamed to __llvm_bb_addr_map) section use a special section type (SHT_LLVM_BB_ADDR_MAP) instead of SHT_PROGBITS. This would help parsers, dumpers and other tools to use the sh_type ELF field to identify this section rather than relying on string comparison on the section name.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D88199