Add support for an external interpreter

The new interface can be called by an external library like ROOT, that manages it's own `TInterpreter` instance.
In this case the `cling::Interpreter*` initialised by TCling is passed to InterOp and a flag indicates that InterOp does not have ownership

include/clang/Interpreter/CppInterOp.h

@@ -530,6 +530,13 @@ namespace Cpp {
   ///\returns the current interpreter instance, if any.
   CPPINTEROP_API TInterp_t GetInterpreter();
 
+  /// Sets the Interpreter instance with an external interpreter, meant to
+  /// be called by an external library that manages it's own interpreter.
+  /// Sets a flag signifying CppInterOp does not have ownership of the
+  /// sInterpreter.
+  ///\param[in] Args - the pointer to an external interpreter
+  CPPINTEROP_API void UseExternalInterpreter(TInterp_t I);
+
   /// Adds a Search Path for the Interpreter to get the libraries.
   CPPINTEROP_API void AddSearchPath(const char* dir, bool isUser = true,
                                     bool prepend = false);

lib/Interpreter/CppInterOp.cpp

@@ -60,19 +60,22 @@ namespace Cpp {
   using namespace llvm;
   using namespace std;
 
-  static std::unique_ptr<compat::Interpreter> sInterpreter;
+  // Flag to indicate ownership when an external interpreter instance is used.
+  static bool OwningSInterpreter = true;
+  static compat::Interpreter* sInterpreter = nullptr;
   // Valgrind complains about __cxa_pure_virtual called when deleting
   // llvm::SectionMemoryManager::~SectionMemoryManager as part of the dtor chain
   // of the Interpreter.
   // This might fix the issue https://reviews.llvm.org/D107087
   // FIXME: For now we just leak the Interpreter.
   struct InterpDeleter {
-    ~InterpDeleter() { sInterpreter.release(); }
+    ~InterpDeleter() {}
   } Deleter;
 
   static compat::Interpreter& getInterp() {
-    assert(sInterpreter.get() && "Must be set before calling this!");
-    return *sInterpreter.get();
+    assert(sInterpreter &&
+           "Interpreter instance must be set before calling this!");
+    return *sInterpreter;
   }
   static clang::Sema& getSema() { return getInterp().getCI()->getSema(); }
   static clang::ASTContext& getASTContext() { return getSema().getASTContext(); }
@@ -2691,12 +2694,16 @@ namespace Cpp {
     // FIXME: Enable this assert once we figure out how to fix the multiple
     // calls to CreateInterpreter.
     //assert(!sInterpreter && "Interpreter already set.");
-    sInterpreter.reset(I);
+    sInterpreter = I;
     return I;
   }
 
-  TInterp_t GetInterpreter() {
-    return sInterpreter.get();
+  TInterp_t GetInterpreter() { return sInterpreter; }
+
+  void UseExternalInterpreter(TInterp_t I) {
+    assert(sInterpreter && "sInterpreter already in use!");
+    sInterpreter = static_cast<compat::Interpreter*>(I);
+    OwningSInterpreter = false;
   }
 
   void AddSearchPath(const char *dir, bool isUser,