[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