[Object/ELF] Add support for setVisibility()

[davide]

Summary

Simon introduced getVisibility() a while ago but now we need setVisibility() in order to implement visibility for ELF symbols in lld.

[atanasyan]

include/llvm/Object/ELFTypes.h

199

The st_other field might contain other flags except STV_xxx. If one sets such flags and then call setVisibility its overwrites the st_other field completely. Maybe write this function something like this:

STV_MASK = 3;

void setVisibility(unsigned char v) {
    assert(v < 4 && "Invalid value for visibility");
    st_other = (st_other & ~STV_MASK) | v;
}

[davide]

You're right, my bad. I updated the patch. When I was looking at this file I noticed that there's a fair amount of magic hardcoded in there. I left SVT_MASK spelled at 0x3 and I'll plan to audit it entirely and change all the hardcoded values with properly defined constants. Does this make sense to you?

[atanasyan]

That sounds like a good plan.

include/llvm/Object/ELFTypes.h

   }
 
   /// Access to the STV_xxx flag stored in the first two bits of st_other.
+  /// STV_DEFAULT: 0
+  /// STV_INTERNAL: 1
+  /// STV_HIDDEN: 2
+  /// STV_PROTECTED: 3
   unsigned char getVisibility() const { return st_other & 0x3; }
+  void setVisibility(unsigned char v) {
+    assert(v < 4 && "Invalid value for visibility");
+    st_other = (st_other & ~0x3) | v;
+  }
 };
 
 /// Elf_Versym: This is the structure of entries in the SHT_GNU_versym section