Unshift the GDB index/GNU pubnames constants modified in r191025

Based on code review feedback from Eric Christopher, unshifting these
constants as they can appear in the gdb_index itself, shifted a further
24 bits. This means that keeping them preshifted is a bit inflexible, so
let's not do that.

Given the motivation, wrap up some nicer enums, more type safety, and
some utility functions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191035 91177308-0d34-0410-b5e6-96231b3b80d8