Add a callback for start/finish code generation.

When we are generating code, CodeGen automatically tries to complete decl's
redeclaration chain. This ends up a call to the external sources, one of which
is our global module index (GMI).

The semantics of the GMI is to aid the frontend, that is to automatically
import missing modules based on the typed *by the user* identifiers. It does
not intend to aid the code generation by any means. Currently this happens
when completing chains for existing identifiers such as 'volume' which happens
to be an indentifier part of TMVA.pcm, too. Thus, CodeGen unintentionally loads
the module of TMVA. This is an overkill, but worse -- it brings recursiveness
to the identifier resolution system.

This patch disables looking in the GMI at codegen time.

include/cling/Interpreter/InterpreterCallbacks.h

@@ -141,6 +141,20 @@ namespace cling {
     ///
     virtual void TransactionCommitted(const Transaction&) {}
 
+    /// This callback is invoked whenever interpreter has started code
+    /// generation for the transaction.
+    ///
+    ///\param[in] - The transaction that is being codegen-ed.
+    ///
+    virtual void TransactionCodeGenStarted(const Transaction&) {}
+
+    /// This callback is invoked whenever interpreter has finished code
+    /// generation for the transaction.
+    ///
+    ///\param[in] - The transaction that is being codegen-ed.
+    ///
+    virtual void TransactionCodeGenFinished(const Transaction&) {}
+
     ///\brief This callback is invoked whenever interpreter has reverted a
     /// transaction that has been fully committed.
     ///

lib/Interpreter/IncrementalParser.cpp

@@ -657,6 +657,9 @@ namespace cling {
     // This llvm::Module is done; finalize it and pass it to the execution
     // engine.
     if (!T->isNestedTransaction() && hasCodeGenerator()) {
+      if (InterpreterCallbacks* callbacks = m_Interpreter->getCallbacks())
+        callbacks->TransactionCodeGenStarted(*T);
+
       // The initializers are emitted to the symbol "_GLOBAL__sub_I_" + filename.
       // Make that unique!
       deserT = beginTransaction(CompilationOptions());
@@ -678,6 +681,9 @@ namespace cling {
         Diags.getClient()->clear();
       }
 
+      if (InterpreterCallbacks* callbacks = m_Interpreter->getCallbacks())
+        callbacks->TransactionCodeGenFinished(*T);
+
       // Create a new module.
       StartModule();
     }

lib/Interpreter/MultiplexInterpreterCallbacks.h

@@ -83,6 +83,18 @@ namespace cling {
        }
      }
 
+     void TransactionCodeGenStarted(const Transaction& T) override {
+       for (auto&& cb : m_Callbacks) {
+         cb->TransactionCodeGenStarted(T);
+       }
+     }
+
+     void TransactionCodeGenFinished(const Transaction& T) override {
+       for (auto&& cb : m_Callbacks) {
+         cb->TransactionCodeGenFinished(T);
+       }
+     }
+
      bool LibraryLoadingFailed(const std::string& errmessage, const std::string& libStem, bool permanent,
          bool resolved) override {
        for (auto&& cb : m_Callbacks) {