Add test for r263138.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@263155 91177308-0d34-0410-b5e6-96231b3b80d8

test/SemaCXX/switch-implicit-fallthrough-off-by-default.cpp

@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 -DUNREACHABLE=1 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 -DUNREACHABLE=0 -Wimplicit-fallthrough %s
+
+void fallthrough(int n) {
+  switch (n) {
+  case 1:
+    if (UNREACHABLE)
+      return;
+    [[fallthrough]]; // expected-no-diagnostics, only checked when UNREACHABLE=0
+  case 2:
+    break;
+  }
+}