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/AsmParser/LLLexer.cpp b/llvm/lib/AsmParser/LLLexer.cpp
index 645d9aa..54c4c59 100644
--- a/llvm/lib/AsmParser/LLLexer.cpp
+++ b/llvm/lib/AsmParser/LLLexer.cpp
@@ -665,6 +665,7 @@
   KEYWORD(sspstrong);
   KEYWORD(strictfp);
   KEYWORD(safestack);
+  KEYWORD(shadowcallstack);
   KEYWORD(sanitize_address);
   KEYWORD(sanitize_hwaddress);
   KEYWORD(sanitize_thread);