[LLD] [COFF] Fix automatically importing data symbols from DLLs with LTO

This broke in 51dcb292, "[lld-link] diagnose undefined symbols
before LTO when possible" (very soon after the 9.0 branch, so
luckily the 9.0 release is unaffected).

The code for loading objects we believe might be needed for autoimport
(loadMinGWAutomaticImports()) does run before the new
reportUnresolvable() function, but it had a condition to only operate
on symbols from regular object files. This condition came from
resolveRemainingUndefines(), but as loadMinGWAutomaticImports() now
has to operate before the LTO, it has to operate on undefineds from
LTO objects as well.

Differential Revision: https://reviews.llvm.org/D70166