This adds functionality for undefined atoms from dynamic libraries to be added
to the list of undefined atoms. 

The processing of undefined atoms from dynamic libraries is controlled by
use-shlib-undefines command line option. 

This patch also adds additional command line arguments to allow/disallow
unresolved symbols from shared libraries and mimics GNU ld behavior.



git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@179257 91177308-0d34-0410-b5e6-96231b3b80d8