[LLD] Improve reporting unresolved symbols in shared libraries

Currently, when reporting unresolved symbols in shared libraries, if an
undefined symbol is firstly seen in a regular object file that shadows
the reference for the same symbol in a shared object. As a result, the
error for the unresolved symbol in the shared library is not reported.
If referencing sections in regular object files are discarded because of
'--gc-sections', no reports about such symbols are generated, and the
linker finishes successfully, generating an output image that fails on
the run.

The patch fixes the issue by keeping symbols, which should be checked,
for each shared library separately.

Differential Revision: https://reviews.llvm.org/D101996