Switch the TargetMachine back to being IGM-specific instead of global to the IRGenerator.

My understanding is that this *should* be read-only, but there
are test cases that are failing that suggest it might not be.

lib/IRGen/IRGenModule.cpp

@@ -114,19 +114,22 @@ static clang::CodeGenerator *createClangCodeGenerator(ASTContext &Context,
   return ClangCodeGen;
 }
 
-IRGenModule::IRGenModule(IRGenerator &irgen, SourceFile *SF,
-                         ASTContext &Context,
+IRGenModule::IRGenModule(IRGenerator &irgen,
+                         std::unique_ptr<llvm::TargetMachine> &&target,
+                         SourceFile *SF,
                          llvm::LLVMContext &LLVMContext,
                          StringRef ModuleName,
                          StringRef OutputFilename)
-  : Context(Context),
+  : IRGen(irgen),
+    Context(irgen.SIL.getASTContext()),
     ClangCodeGen(createClangCodeGenerator(Context, LLVMContext,
                                           irgen.Opts, ModuleName)),
     Module(*ClangCodeGen->GetModule()),
     LLVMContext(Module.getContext()),
-    DataLayout(irgen.TargetMachine->createDataLayout()),
+    DataLayout(target->createDataLayout()),
     Triple(Context.LangOpts.Target),
-    OutputFilename(OutputFilename), IRGen(irgen),
+    TargetMachine(std::move(target)),
+    OutputFilename(OutputFilename),
     TargetInfo(SwiftTargetInfo::get(*this)),
     DebugInfo(0), ModuleHash(nullptr),
     ObjCInterop(Context.LangOpts.EnableObjCInterop),