[ELF] Rename LazyObject to LazySymbol. NFC

LazySymbol (used by wasm port) is a more accurate name (the struct is
not about an object). However, the ELF port calls this LazyObject likely
because we used to have LazyArchive (removed by
https://reviews.llvm.org/D119074), and LazyObject has a similar naming
convention (LazyObjectSymbol would be better, but it is too long).

Reviewers: dschuff

Pull Request: https://github.com/llvm/llvm-project/pull/78809