Add the ShadowCallStack attribute

Summary:
Introduce the ShadowCallStack function attribute. It's added to
functions compiled with -fsanitize=shadow-call-stack in order to mark
functions to be instrumented by a ShadowCallStack pass to be submitted
in a separate change.

Reviewers: pcc, kcc, kubamracek

Reviewed By: pcc, kcc

Subscribers: cryptoad, mehdi_amini, javed.absar, llvm-commits, kcc

Differential Revision: https://reviews.llvm.org/D44800

llvm-svn: 329108
diff --git a/llvm/lib/IR/Verifier.cpp b/llvm/lib/IR/Verifier.cpp
index 4cb1661..92bb53b 100644
--- a/llvm/lib/IR/Verifier.cpp
+++ b/llvm/lib/IR/Verifier.cpp
@@ -1413,6 +1413,7 @@
   case Attribute::StackProtectReq:
   case Attribute::StackProtectStrong:
   case Attribute::SafeStack:
+  case Attribute::ShadowCallStack:
   case Attribute::NoRedZone:
   case Attribute::NoImplicitFloat:
   case Attribute::Naked: