[BOLT] Make .debug_loc update deterministic

Summary:
Change the single DebugLocWriter to one for each compilation unit. Then, each thread can write to its own DebugLocWriter and we can combine the data in a deterministic order once the threads are done.

The only catch is that each thread would need the offset of the location lists it adds, so we make a list of pending location list patches and compute the final offsets at the end.

(cherry picked from FBD18153069)