[ELF] Add -z muldefs option.

This adds -z muldefs option which is widely used over
--allow-multiple-definition.

This option is supported by the GNU linker.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205391 91177308-0d34-0410-b5e6-96231b3b80d8
[ELF] Create Attribute class associated with Input files.

The attribute class holds positional attributes for Input files specified on the
command line for the Gnu flavor.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205392 91177308-0d34-0410-b5e6-96231b3b80d8