lld: do not report undefined weak references in shared libraries

lld: do not report undefined weak references in shared libraries

https://github.com/llvm/llvm-project/commit/52bfd2c1ccd86ff813ee6df5a6700690acdd912f

This fixes an issue introduced in D101996.

A weak reference in a shared library could be incorrectly reported if
there is another library that has a strong reference to the same symbol.

Differential Revision: https://reviews.llvm.org/D115041

ok patrick@ kettenis@