[lld-link] Replace LazyObjFile with lazy ObjFile/BitcodeFile

[lld-link] Replace LazyObjFile with lazy ObjFile/BitcodeFile

Similar to ELF 3a5fb57393c3bc77be9e7afc2ec9d4ec3c9bbf70.

* previously when a LazyObjFile was extracted, a new ObjFile/BitcodeFile was created; now the file is reused, just with `lazy` cleared
* avoid the confusing transfer of `symbols` from LazyObjFile to the new file
* simpler code, smaller executable (5200+ bytes smaller on x86-64)
* make eager parsing feasible (for parallel section/symbol table initialization)

Reviewed By: aganea, rnk

Differential Revision: https://reviews.llvm.org/D116434