[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