add ::drop_back() and ::drop_front() methods, which are like pop_front/pop_back on a vector, but a) aren't destructive to "this", and b) can take a # elements to drop.

llvm-svn: 148791

include/llvm/ADT/StringRef.h

@@ -353,6 +353,20 @@ namespace llvm {
       Start = min(Start, Length);
       return StringRef(Data + Start, min(N, Length - Start));
     }
+    
+    /// drop_front - Return a StringRef equal to 'this' but with the first
+    /// elements dropped.
+    StringRef drop_front(unsigned N = 1) const {
+      assert(size() >= N && "Dropping more elements than exist");
+      return substr(N);
+    }
+
+    /// drop_back - Return a StringRef equal to 'this' but with the last
+    /// elements dropped.
+    StringRef drop_back(unsigned N = 1) const {
+      assert(size() >= N && "Dropping more elements than exist");
+      return substr(0, size()-N);
+    }
 
     /// slice - Return a reference to the substring from [Start, End).
     ///