[lld] [LLD][COFF] Create COFFObjectFile instance when constructing ObjFile (NFC) (PR #120144)

[Jacek Caban via llvm-commits]

cjacek wrote:

I benchmarked this change by cross-compiling `clang.exe` with llvm-mingw, and the difference was within the margin of error. I also checked `-start-lib`/`-end-lib` code path by hacking a script to use that for linking `clang.exe` and observed similar results. According to `perf report`, `COFFObjectFile::create` accounts for approximately 0.01% of execution time, and the change does not affect measured peak memory usage.

https://github.com/llvm/llvm-project/pull/120144