[lld-macho] Split symbol table initialization and resolution; cache InputFile::symbols for --start-lib

[MaskRay]

Symbol table initialization (populating InputFile::symbol[*]) is easier to parallelize than symbol resolution.

See https://reviews.llvm.org/D62188 https://reviews.llvm.org/D62290 from ELF.

Then, in the --start-lib (#52931) state, InputFile::symbols can be saved to be reused by non-lazy ObjFile::parse BitcodeFile::parse (see D116390).

Future direction: introduce a mode to do eager parsing. The prerequisite is to replace ArchiveFile with lazy ObjFile/BitcodeFile.

[llvmbot]

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