[LLD][COFF] PDB: Parallel sort publics

Saves up to 1.3 sec on large PDBs.
Figures below are for the "Globals Stream Layout" pass:

                            Before	This patch
Large EXE (PDB is ~2 GB)	3330 ms	2022 ms
Large EXE (PDB is ~2 GB)	2680 ms	1608 ms
Large DLL (PDB is ~1 GB)	1455 ms	938 ms
Large DLL (PDB is ~800 MB)	1215 ms	800 ms
Small DLL (PDB is ~200 MB)	224 ms	146 ms

Differential Revision: https://reviews.llvm.org/D56334

llvm-svn: 350452