New version of StringRef brings a bunch of includes in etc.

Instead of copying over more includes, outline the only function in the
Interpreter which requires a full definition of llvm::StringRef.

include/cling/Interpreter/Interpreter.h

@@ -12,8 +12,6 @@
 
 #include "cling/Interpreter/InvocationOptions.h"
 
-#include "llvm/ADT/StringRef.h"
-
 #include <cstdlib>
 #include <memory>
 #include <string>
@@ -25,6 +23,7 @@ namespace llvm {
   class ExecutionEngine;
   class LLVMContext;
   class Module;
+  class StringRef;
   class Type;
   template <typename T> class SmallVectorImpl;
 }
@@ -380,9 +379,7 @@ namespace cling {
 
     ///\brief Adds a single include path (-I).
     ///
-    void AddIncludePath(llvm::StringRef PathsStr) {
-      return AddIncludePaths(PathsStr, nullptr);
-    }
+     void AddIncludePath(llvm::StringRef PathsStr);
 
     ///\brief Prints the current include paths that are used.
     ///

lib/Interpreter/Interpreter.cpp

@@ -477,6 +477,10 @@ namespace cling {
     }
   }
 
+  void Interpreter::AddIncludePath(llvm::StringRef PathsStr) {
+    return AddIncludePaths(PathsStr, nullptr);
+  }
+
   void Interpreter::DumpIncludePath(llvm::raw_ostream* S) {
     utils::DumpIncludePaths(getCI()->getHeaderSearchOpts(), S ? *S : cling::outs(),
                             true /*withSystem*/, true /*withFlags*/);