[dsymutil] Introduce LinkContext. NFC.

This patch introduces the LinkContext which is necessary to have
dsymutil perform analysis and cloning of DIEs in parallel. As requested
in D43945, I'm landing this as two separate commits.

llvm-svn: 327382

tools/dsymutil/DebugMap.h

@@ -102,6 +102,8 @@ public:
 
   const_iterator end() const { return Objects.end(); }
 
+  unsigned getNumberOfObjects() const { return Objects.size(); }
+
   /// This function adds an DebugMapObject to the list owned by this
   /// debug map.
   DebugMapObject &