Reland D119909 [ELF] Parallelize initializeLocalSymbols

ObjFile::parse combines symbol initialization and resolution. Many tasks
unrelated to symbol resolution can be postponed and parallelized. This patch
extracts local symbol initialization and parallelizes it.

Technically the new function initializeLocalSymbols can be merged into
ObjFile::postParse, but functions like getSrcMsg may access the
uninitialized (all nullptr) local part of InputFile::symbols.

Linking chrome: 1.02x as fast with glibc malloc, 1.04x as fast with mimalloc

Depends on f456c3ae and D119908

Reviewed By: ikudrin

Differential Revision: https://reviews.llvm.org/D119909