Adding support for --unresolved-symbols option for ELF file format in llvm linker lld.
Details of this option can be found in gnu ld(1) man page.
Implemented all four values of this option: report-all, ignore-all, ignore-in-object-files and ignore-in-shared-libs
Implemented behavior of all the values as standalone as well as in combination with options --allow-shlib-undefined, --no-allow-shlib-undefined, -shared, -static
Patch by Shridhar Joshi