[ELF] Parallelize input section initialization

This implements the last step of
https://discourse.llvm.org/t/parallel-input-file-parsing/60164 for the ELF port.

For an ELF object file, we previously did: parse, (parallel) initializeLocalSymbols, (parallel) postParseObjectFile.
Now we do: parse, (parallel) initSectionsAndLocalSyms, (parallel) postParseObjectFile.

initSectionsAndLocalSyms does most of input section initialization.
The sequential `parse` does SHT_ARM_ATTRIBUTES/SHT_RISCV_ATTRIBUTES/SHT_GROUP initialization for now.

Performance linking some programs with --threads=8 (glibc 2.33 malloc and mimalloc):

* clang: 1.05x as fast with glibc malloc, 1.03x as fast with mimalloc
* chrome: 1.04x as fast with glibc malloc, 1.03x as fast with mimalloc
* internal search program: 1.08x as fast with glibc malloc, 1.05x as fast with mimalloc

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D130810