[clang-repl] Enable debugging of JIT-ed code.

This change follows 21b5ebd and makes use of the jitlink
infrastructure. In order to use this feature inside lldb one needs to run the
lldb command: settings set plugin.jit-loader.gdb.enable on

This works currently only on Darwin since jitlink is not a default ELF/x86-64
backend yet.

Differential revision: https://reviews.llvm.org/D148481

clang/lib/Interpreter/IncrementalExecutor.cpp

@@ -21,11 +21,18 @@
 #include "llvm/ExecutionEngine/Orc/IRCompileLayer.h"
 #include "llvm/ExecutionEngine/Orc/LLJIT.h"
 #include "llvm/ExecutionEngine/Orc/RTDyldObjectLinkingLayer.h"
+#include "llvm/ExecutionEngine/Orc/TargetProcess/JITLoaderGDB.h"
 #include "llvm/ExecutionEngine/SectionMemoryManager.h"
 #include "llvm/IR/Module.h"
 #include "llvm/Support/ManagedStatic.h"
 #include "llvm/Support/TargetSelect.h"
 
+// Force linking some of the runtimes that helps attaching to a debugger.
+LLVM_ATTRIBUTE_USED void linkComponents() {
+  llvm::errs() << (void *)&llvm_orc_registerJITLoaderGDBWrapper
+               << (void *)&llvm_orc_registerJITLoaderGDBAllocAction;
+}
+
 namespace clang {
 
 IncrementalExecutor::IncrementalExecutor(llvm::orc::ThreadSafeContext &TSC,
@@ -37,7 +44,12 @@ IncrementalExecutor::IncrementalExecutor(llvm::orc::ThreadSafeContext &TSC,
 
   auto JTMB = JITTargetMachineBuilder(TI.getTriple());
   JTMB.addFeatures(TI.getTargetOpts().Features);
-  if (auto JitOrErr = LLJITBuilder().setJITTargetMachineBuilder(JTMB).create())
+  LLJITBuilder Builder;
+  Builder.setJITTargetMachineBuilder(JTMB);
+  // Enable debugging of JIT'd code (only works on JITLink for ELF and MachO).
+  Builder.setEnableDebuggerSupport(true);
+
+  if (auto JitOrErr = Builder.create())
     Jit = std::move(*JitOrErr);
   else {
     Err = JitOrErr.takeError();