[ELF] Fix interaction between --unresolved-symbols= and --[no-]allow-shlib-undefined

As mentioned in https://reviews.llvm.org/D67479#1667256 ,

* `--[no-]allow-shlib-undefined` control the diagnostic for an unresolved symbol in a shared object
* `-z defs/-z undefs` control the diagnostic for an unresolved symbol in a regular object file
* `--unresolved-symbols=` controls both bits.

In addition, make --warn-unresolved-symbols affect --no-allow-shlib-undefined.

This patch makes the behavior match GNU ld.

Reviewed By: psmith

Differential Revision: https://reviews.llvm.org/D91510