Add !associated metadata.

This is an ELF-specific thing that adds SHF_LINK_ORDER to the global's section
pointing to the metadata argument's section. The effect of that is a reverse dependency
between sections for the linker GC.

!associated does not change the behavior of global-dce. The global
may also need to be added to llvm.compiler.used.

Since SHF_LINK_ORDER is per-section, !associated effectively enables
fdata-sections for the affected globals, the same as comdats do.

Differential Revision: https://reviews.llvm.org/D29104

llvm-svn: 298157