ELF: Read address significance tables with --icf=all.

Under --icf=all we now only apply KeepUnique to non-executable
address-significant sections. This has the effect of making --icf=all
mean unsafe ICF for executable sections and safe ICF for non-executable
sections.

With this change the meaning of the KeepUnique bit changes to
"does the current ICF mode (together with the --keep-unique and
--ignore-data-address-equality flags) require this section to be
kept unique".

Differential Revision: https://reviews.llvm.org/D49626

llvm-svn: 337640