[PATCH] [Object/ELF] Add support for setVisibility()

[Simon Atanasyan]

REPOSITORY
  rL LLVM

================
Comment at: include/llvm/Object/ELFTypes.h:199
@@ +198,3 @@
+    assert(v < 4 && "Invalid value for visibility");
+    st_other &= v;
+  }
----------------
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;
}
```

http://reviews.llvm.org/D8279

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