[Orc] Add NotifyCreated callback for LLJITBuilder (#84175)

This is useful to attach generators to JITDylibs or inject initial
symbol definitions.

llvm/include/llvm/ExecutionEngine/Orc/LLJIT.h

@@ -311,6 +311,8 @@ class LLJITBuilderState {
 
   using PlatformSetupFunction = unique_function<Expected<JITDylibSP>(LLJIT &J)>;
 
+  using NotifyCreatedFunction = std::function<Error(LLJIT &)>;
+
   std::unique_ptr<ExecutorProcessControl> EPC;
   std::unique_ptr<ExecutionSession> ES;
   std::optional<JITTargetMachineBuilder> JTMB;
@@ -321,6 +323,7 @@ class LLJITBuilderState {
   CompileFunctionCreator CreateCompileFunction;
   unique_function<Error(LLJIT &)> PrePlatformSetup;
   PlatformSetupFunction SetUpPlatform;
+  NotifyCreatedFunction NotifyCreated;
   unsigned NumCompileThreads = 0;
 
   /// Called prior to JIT class construcion to fix up defaults.
@@ -441,6 +444,16 @@ class LLJITBuilderSetters {
     return impl();
   }
 
+  /// Set up a callback after successful construction of the JIT.
+  ///
+  /// This is useful to attach generators to JITDylibs or inject initial symbol
+  /// definitions.
+  SetterImpl &
+  setNotifyCreatedCallback(LLJITBuilderState::NotifyCreatedFunction Callback) {
+    impl().NotifyCreated = std::move(Callback);
+    return impl();
+  }
+
   /// Set the number of compile threads to use.
   ///
   /// If set to zero, compilation will be performed on the execution thread when
@@ -474,6 +487,11 @@ class LLJITBuilderSetters {
     std::unique_ptr<JITType> J(new JITType(impl(), Err));
     if (Err)
       return std::move(Err);
+
+    if (impl().NotifyCreated)
+      if (Error Err = impl().NotifyCreated(*J))
+        return Err;
+
     return std::move(J);
   }