[MC] Implement EmitRawText in MCNullStreamer

[eush]

Summary

This adds dummy implementation of EmitRawText in MCNullStreamer.

This fixes the behavior of AsmPrinter with MCNullStreamer on targets
on which no integrated assembler is used. An attempt to emit inline asm
on such a target would previously lead to a crash, since AsmPrinter does not
check for hasRawTextSupport in EmitInlineAsm and calls EmitRawText
anyway if integrated assembler is disabled (the behavior has changed
in D2686).

Error message printed by MCStreamer:

EmitRawText called on an MCStreamer that doesn't support it, something
must not be fully mc'ized

Patch by Eugene Sharygin

[dsanders]

LGTM. It makes sense to me that MCNullStreamer supports raw text by not emitting anything.

[eush]

Could you commit this patch on my behalf?

[dsanders]

In D53938#1283840, @eush wrote:

Could you commit this patch on my behalf?

Sure. It's r345841

lib/MC/MCNullStreamer.cpp

 //===- lib/MC/MCNullStreamer.cpp - Dummy Streamer Implementation ----------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
+#include "llvm/ADT/StringRef.h"
 #include "llvm/MC/MCInst.h"
 #include "llvm/MC/MCStreamer.h"
 #include "llvm/MC/MCSymbol.h"
 
 using namespace llvm;
 
 namespace {
 
   class MCNullStreamer : public MCStreamer {
   public:
     MCNullStreamer(MCContext &Context) : MCStreamer(Context) {}
 
     /// @name MCStreamer Interface
     /// @{
 
+    bool hasRawTextSupport() const override { return true; }
+    void EmitRawTextImpl(StringRef String) override {}
+
     bool EmitSymbolAttribute(MCSymbol *Symbol,
                              MCSymbolAttr Attribute) override {
       return true;
     }
 
     void EmitCommonSymbol(MCSymbol *Symbol, uint64_t Size,
                           unsigned ByteAlignment) override {}
     void EmitZerofill(MCSection *Section, MCSymbol *Symbol = nullptr,

test/CodeGen/Hexagon/inline-asm-filetype-null.ll

+; RUN: llc -filetype=null < %s
+
+target triple = "hexagon"
+
+define void @foo() {
+  tail call void asm sideeffect "//", ""()
+  ret void
+}