Add llvm.codeview.annotation to implement MSVC __annotation

Summary:
This intrinsic represents a label with a list of associated metadata
strings. It is modelled as reading and writing inaccessible memory so
that it won't be removed as dead code. I think the intention is that the
annotation strings should appear at most once in the debug info, so I
marked it noduplicate. We are allowed to inline code with annotations as
long as we strip the annotation, but that can be done later.

Reviewers: majnemer

Subscribers: eraman, llvm-commits, hiraditya

Differential Revision: https://reviews.llvm.org/D36904

llvm-svn: 312569
diff --git a/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.h b/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.h
index fe4f0a0..b87b1ce 100644
--- a/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.h
+++ b/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.h
@@ -119,6 +119,8 @@
 
     SmallVector<LocalVariable, 1> Locals;
 
+    std::vector<std::pair<MCSymbol *, MDNode *>> Annotations;
+
     const MCSymbol *Begin = nullptr;
     const MCSymbol *End = nullptr;
     unsigned FuncId = 0;