[clang][tests] Add a testcase for C++ DR2406

[tbaeder]

Summary

DR2406 is in "unknown" state according to https://clang.llvm.org/cxx_dr_status.html.

The described defects do not exist in clang. The diagnostic messages are different but the program is still recognized as ill-formed.

Add a testcase in clang/test/CXX/drs. If I understand the make_cxx_dr_status script correctly, this should update the above webpage.

[aaron.ballman]

LGTM, thank you for spotting this!

clang/test/CXX/drs/dr2406.cpp

+// RUN: %clang_cc1 -x c++ %s  -verify
+
+// dr2406: yes
+
+void fallthrough(int n) {
+  void g(), h(), i();
+  switch (n) {
+  case 1:
+  case 2:
+    g();
+    [[fallthrough]];
+  case 3: // warning on fallthrough discouraged
+    do {
+      [[fallthrough]]; // expected-error {{fallthrough annotation does not directly precede switch label}}
+    } while (false);
+  case 6:
+    do {
+      [[fallthrough]]; // expected-error {{fallthrough annotation does not directly precede switch label}}
+    } while (n);
+  case 7:
+    while (false) {
+      [[fallthrough]]; // expected-error {{fallthrough annotation does not directly precede switch label}}
+    }
+  case 5:
+    h();
+  case 4: // implementation may warn on fallthrough
+    i();
+    [[fallthrough]]; // expected-error {{fallthrough annotation does not directly precede switch label}}
+  }
+}