[IR] Remove unused declaration InitializeTypeMap (NFC)

The corresponding function definition was removed on Apr 23, 2016 in
commit a59d3e5af8.

llvm/include/llvm/IR/DebugInfo.h

@@ -106,8 +106,6 @@ public:
   void reset();
 
 private:
-  void InitializeTypeMap(const Module &M);
-
   void processCompileUnit(DICompileUnit *CU);
   void processScope(DIScope *Scope);
   void processType(DIType *DT);