[yaml2obj/obj2yaml] - Add support for SHT_LLVM_ADDRSIG sections.

SHT_LLVM_ADDRSIG is described here:
https://llvm.org/docs/Extensions.html#sht-llvm-addrsig-section-address-significance-table

This patch teaches tools to dump them and to parse the YAML declarations of such sections.

Differential revision: https://reviews.llvm.org/D68333

llvm-svn: 373598