[ORC][LLJIT] Add a Pre-PlatformSetup-Setup function.

This function will be run prior to platform setup to provide LLJIT clients with
a chance to customize the LLJIT instance (e.g. install plugins) before the JIT
runtime is loaded.

The motivating use-case is debugger support: We want to install the debugger
plugin before the runtime is loaded (during platform setup) so that the runtime
itself can be debugged. A patch to do this will be committed shortly.

Author: lhames

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

@@ -319,6 +319,7 @@ class LLJITBuilderState {
   ProcessSymbolsJITDylibSetupFunction SetupProcessSymbolsJITDylib;
   ObjectLinkingLayerCreator CreateObjectLinkingLayer;
   CompileFunctionCreator CreateCompileFunction;
+  unique_function<Error(LLJIT &)> PrePlatformSetup;
   PlatformSetupFunction SetUpPlatform;
   unsigned NumCompileThreads = 0;
   bool EnableDebuggerSupport = false;
@@ -418,6 +419,19 @@ class LLJITBuilderSetters {
     return impl();
   }
 
+  /// Set a setup function to be run just before the PlatformSetupFunction is
+  /// run.
+  ///
+  /// This can be used to customize the LLJIT instance before the platform is
+  /// set up. E.g. By installing a debugger support plugin before the platform
+  /// is set up (when the ORC runtime is loaded) we enable debugging of the
+  /// runtime itself.
+  SetterImpl &
+  setPrePlatformSetup(unique_function<Error(LLJIT &)> PrePlatformSetup) {
+    impl().PrePlatformSetup = std::move(PrePlatformSetup);
+    return impl();
+  }
+
   /// Set up an PlatformSetupFunction.
   ///
   /// If this method is not called then setUpGenericLLVMIRPlatform

llvm/lib/ExecutionEngine/Orc/LLJIT.cpp

@@ -1054,6 +1054,13 @@ LLJIT::LLJIT(LLJITBuilderState &S, Error &Err)
     }
   }
 
+  if (S.PrePlatformSetup) {
+    if (auto Err2 = S.PrePlatformSetup(*this)) {
+      Err = std::move(Err2);
+      return;
+    }
+  }
+
   if (!S.SetUpPlatform)
     S.SetUpPlatform = setUpGenericLLVMIRPlatform;