[BOLT][DWARF] Refactor of Loc and LocLists writers

Summary:
Refactored Loc and LocList writers to write out entries during finalization phase,
 and hid some of the details in a class.
This simplifies things from impelementation details, and also will be needed for
DWARF5 where we need to know how many locLists entries there are there.

(cherry picked from FBD31563795)