Merge from mainline.

Properly handle linking of strong alias with weak function, this fixes
      PR2883


git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_24@57707 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Linker/LinkModules.cpp

@@ -984,7 +984,7 @@ static bool LinkFunctionProtos(Module *Dest, const Module *Src,
       // The only valid mappings are:
       // - SF is external declaration, which is effectively a no-op.
       // - SF is weak, when we just need to throw SF out.
-      if (!SF->isDeclaration())
+      if (!SF->isDeclaration() && !SF->mayBeOverridden())
         return Error(Err, "Function-Alias Collision on '" + SF->getName() +
                      "': symbol multiple defined");
     }