Try to fix the bots with a forward decl.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184077 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/clang/AST/ASTContext.h b/include/clang/AST/ASTContext.h
index a74b526..830a956 100644
--- a/include/clang/AST/ASTContext.h
+++ b/include/clang/AST/ASTContext.h
@@ -46,6 +46,7 @@
 
 namespace clang {
   class FileManager;
+  class AtomicExpr;
   class ASTRecordLayout;
   class BlockExpr;
   class CharUnits;