[Orc][LLJIT] Re-apply 298e183e813 (use JITLink for LLJIT where supported).

Patch d9220b580b3 fixed the underlying issue that casued 298e183e813 to fail.
diff --git a/llvm/include/llvm/ExecutionEngine/Orc/JITTargetMachineBuilder.h b/llvm/include/llvm/ExecutionEngine/Orc/JITTargetMachineBuilder.h
index c8c4ecd..4f2f557 100644
--- a/llvm/include/llvm/ExecutionEngine/Orc/JITTargetMachineBuilder.h
+++ b/llvm/include/llvm/ExecutionEngine/Orc/JITTargetMachineBuilder.h
@@ -79,12 +79,18 @@
     return *this;
   }
 
+  /// Get the relocation model.
+  const Optional<Reloc::Model> &getRelocationModel() const { return RM; }
+
   /// Set the code model.
   JITTargetMachineBuilder &setCodeModel(Optional<CodeModel::Model> CM) {
     this->CM = std::move(CM);
     return *this;
   }
 
+  /// Get the code model.
+  const Optional<CodeModel::Model> &getCodeModel() const { return CM; }
+
   /// Set the LLVM CodeGen optimization level.
   JITTargetMachineBuilder &setCodeGenOptLevel(CodeGenOpt::Level OptLevel) {
     this->OptLevel = OptLevel;
diff --git a/llvm/lib/ExecutionEngine/Orc/LLJIT.cpp b/llvm/lib/ExecutionEngine/Orc/LLJIT.cpp
index 5c39373..54473ab 100644
--- a/llvm/lib/ExecutionEngine/Orc/LLJIT.cpp
+++ b/llvm/lib/ExecutionEngine/Orc/LLJIT.cpp
@@ -24,11 +24,15 @@
       JTMB = std::move(*JTMBOrErr);
     else
       return JTMBOrErr.takeError();
+  }
 
-    // If no ObjectLinkingLayer creator was set and the target supports JITLink
-    // then configure for JITLink.
+  // If the client didn't configure any linker options then auto-configure the
+  // JIT linker.
+  if (!CreateObjectLinkingLayer && JTMB->getCodeModel() == None &&
+      JTMB->getRelocationModel() == None) {
+
     auto &TT = JTMB->getTargetTriple();
-    if (!CreateObjectLinkingLayer && TT.isOSBinFormatMachO() &&
+    if (TT.isOSBinFormatMachO() &&
         (TT.getArch() == Triple::aarch64 || TT.getArch() == Triple::x86_64)) {
 
       JTMB->setRelocationModel(Reloc::PIC_);