Fix crash when trying to pack-expand a GNU statement expression.

We could in principle support such pack expansion, using techniques similar to
what we do for pack expansion of lambdas, but it's not clear it's worthwhile.
For now at least, cleanly reject these cases rather than crashing.

llvm-svn: 324160
diff --git a/clang/lib/Sema/SemaStmt.cpp b/clang/lib/Sema/SemaStmt.cpp
index 1ebc367..9390703 100644
--- a/clang/lib/Sema/SemaStmt.cpp
+++ b/clang/lib/Sema/SemaStmt.cpp
@@ -337,8 +337,8 @@
   DiagRuntimeBehavior(Loc, nullptr, PDiag(DiagID) << R1 << R2);
 }
 
-void Sema::ActOnStartOfCompoundStmt() {
-  PushCompoundScope();
+void Sema::ActOnStartOfCompoundStmt(bool IsStmtExpr) {
+  PushCompoundScope(IsStmtExpr);
 }
 
 void Sema::ActOnFinishOfCompoundStmt() {