Revert r337638, "ELF: Make sections with KeepUnique bit eligible for ICF."

Revert r337638, "ELF: Make sections with KeepUnique bit eligible for ICF."

The gold behaviour with regard to --keep-unique is arguably a bug.
I also noticed a bug in my patch, which is that we mislink the
following program with --icf=safe by merging f3 and f4:

void f1() {}
void f2() {}

__attribute__((weak)) void* f3() { return f1; }
__attribute__((weak)) void* f4() { return f2; }

int main() {
  printf("%p %p\n", f3(), f4());
}

llvm-svn: 337729