[MC] Implement EmitRawText in MCNullStreamer

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

Reviewers: dsanders, echristo

Reviewed By: dsanders

Subscribers: eraman, llvm-commits

Differential Revision: https://reviews.llvm.org/D53938

llvm-svn: 345841

llvm/lib/MC/MCNullStreamer.cpp

@@ -7,6 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "llvm/ADT/StringRef.h"
 #include "llvm/MC/MCInst.h"
 #include "llvm/MC/MCStreamer.h"
 #include "llvm/MC/MCSymbol.h"
@@ -22,6 +23,9 @@ namespace {
     /// @name MCStreamer Interface
     /// @{
 
+    bool hasRawTextSupport() const override { return true; }
+    void EmitRawTextImpl(StringRef String) override {}
+
     bool EmitSymbolAttribute(MCSymbol *Symbol,
                              MCSymbolAttr Attribute) override {
       return true;

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

@@ -0,0 +1,8 @@
+; RUN: llc -filetype=null < %s
+
+target triple = "hexagon"
+
+define void @foo() {
+  tail call void asm sideeffect "//", ""()
+  ret void
+}