ELF: Do not ICF SHF_LINK_ORDER sections.

We are already ICF'ing these sections as a unit with their dependent
sections, so they don't need to be considered for ICF individually.

This change also "fixes" slowness caused by our quadratic-in-group-size
relocation segregation algorithm on 32-bit ARM platforms with unwind
data and ICF on rodata. In this scenario almost every function's
.ARM.exidx is identical except for the targets of the relocations
that refer to the function and its .ARM.extab, which causes almost
all of the program's .ARM.exidx sections to be initially added to the
same class, which causes us to compare every such section with every
other such section.

Differential Revision: https://reviews.llvm.org/D49716

llvm-svn: 337967