ELF: Allow forward references to linked sections.

It's possible to create IR that uses !associated to refer to a global that
appears later in the module, which can result in these types of forward
references being generated. Unfortunately our assembler does not currently
accept the resulting .s so I needed to use yaml2obj to test this.

Differential Revision: https://reviews.llvm.org/D64880

llvm-svn: 366460