Pacify gcc's -Wreturn-type.

A separate unreachable message will make it easier to debug if either of the
unreachables is reached.

llvm-svn: 149402
diff --git a/clang/lib/AST/Expr.cpp b/clang/lib/AST/Expr.cpp
index 337a31a..fc0b9a7 100644
--- a/clang/lib/AST/Expr.cpp
+++ b/clang/lib/AST/Expr.cpp
@@ -2178,6 +2178,7 @@
   case NoStmtClass:
     llvm_unreachable("Invalid class for expression");
   }
+  llvm_unreachable("Bogus StmtClass");
 }
 
 Expr* Expr::IgnoreParens() {