Use Backend_EmitMCNull for null codegen unit tests.

[dlj]

Summary

Using Backend_EmitLL attemps to create a file with an empty filename.
This is problematic in certain environments: an empty filename may be
illegal, or the default output path may not be writable (in the case
where an empty filename would otherwise have some non-failing
semantics). This patch switches to use Backend_EmitMCNull, which
allows CodeGen to run, but does not attempt to create or write an
output file.

[rsmith]

LGTM

unittests/Frontend/CodeGenActionTest.cpp

 using namespace clang::frontend;
 
 namespace {
 
 
 class NullCodeGenAction : public CodeGenAction {
 public:
   NullCodeGenAction(llvm::LLVMContext *_VMContext = nullptr)
-    : CodeGenAction(Backend_EmitLL, _VMContext) {}
+    : CodeGenAction(Backend_EmitMCNull, _VMContext) {}
 
   // The action does not call methods of ATContext.
   void ExecuteAction() override {
     CompilerInstance &CI = getCompilerInstance();
     if (!CI.hasPreprocessor())
       return;
     if (!CI.hasSema())
       CI.createSema(getTranslationUnitKind(), nullptr);