[llvm-link] Improve link time for bitcode archives [NFC]

Linking large bitcode archives currently takes a lot of time with llvm-link,
this patch adds couple improvements which reduce link time for archives
- Use one Linker instance for archive instead of recreating it for each member
- Lazy load archive members

Reviewed By: tra, jdoerfert

Differential Revision: https://reviews.llvm.org/D94643