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.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@51118 91177308-0d34-0410-b5e6-96231b3b80d8

lib/ExecutionEngine/ExecutionEngine.cpp

@@ -884,7 +884,7 @@ void ExecutionEngine::emitGlobals() {
           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;
       }