Add CommonLinkage; currently tentative definitions
are represented as "weak", but there are subtle differences
in some cases on Darwin, so we need both.  The intent
is that "common" will behave identically to "weak" unless
somebody changes their target to do something else.
No functional change as yet.

llvm-svn: 51118
diff --git a/llvm/lib/ExecutionEngine/ExecutionEngine.cpp b/llvm/lib/ExecutionEngine/ExecutionEngine.cpp
index a56951d..8bd3b11 100644
--- a/llvm/lib/ExecutionEngine/ExecutionEngine.cpp
+++ b/llvm/lib/ExecutionEngine/ExecutionEngine.cpp
@@ -884,7 +884,7 @@
           continue;
         
         // Otherwise, we know it's linkonce/weak, replace it if this is a strong
-        // symbol.
+        // symbol.  FIXME is this right for common?
         if (GV->hasExternalLinkage() || GVEntry->hasExternalWeakLinkage())
           GVEntry = GV;
       }