[JITLink] Fix access of moved-from argument in 265d6bd.

The LinkGraph that is passed into the ELFJITLinker_x86_64 constructor is handed
off to the JITLinker base class, so can't be used in the constructor body. This
commit adds a getGraph method to JITLinker, and updates ELFJITLinker_x86_64 to
use that.

Author: lhames

llvm/lib/ExecutionEngine/JITLink/ELF_x86_64.cpp

@@ -243,7 +243,7 @@ class ELFJITLinker_x86_64 : public JITLinker<ELFJITLinker_x86_64> {
                       PassConfiguration PassConfig)
       : JITLinker(std::move(Ctx), std::move(G), std::move(PassConfig)) {
 
-    if (shouldAddDefaultTargetPasses(G->getTargetTriple()))
+    if (shouldAddDefaultTargetPasses(getGraph().getTargetTriple()))
       getPassConfig().PostAllocationPasses.push_back(
           [this](LinkGraph &G) { return getOrCreateGOTSymbol(G); });
   }

llvm/lib/ExecutionEngine/JITLink/JITLinkGeneric.h

@@ -43,6 +43,9 @@ class JITLinkerBase {
   using AllocResult = Expected<std::unique_ptr<InFlightAlloc>>;
   using FinalizeResult = Expected<JITLinkMemoryManager::FinalizedAlloc>;
 
+  // Returns a reference to the graph being linked.
+  LinkGraph &getGraph() { return *G; }
+
   // Returns true if the context says that the linker should add default
   // passes. This can be used by JITLinkerBase implementations when deciding
   // whether they should add default passes.