[Object][ELF] ELFEntityIterator : Add operators for random access

Add operators add/subtract for random access. This is essentially used by
lld.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232453 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Object/ELF.h

@@ -94,6 +94,18 @@ public:
       return *this;
     }
 
+    ELFEntityIterator &operator+(difference_type n) {
+      assert(Current && "Attempted to increment an invalid iterator!");
+      Current += n;
+      return *this;
+    }
+
+    ELFEntityIterator &operator-(difference_type n) {
+      assert(Current && "Attempted to subtract an invalid iterator!");
+      Current -= n;
+      return *this;
+    }
+
     ELFEntityIterator operator ++(int) {
       ELFEntityIterator Tmp = *this;
       ++*this;