Add fallthrough annotations on new C++ switches

A recent commit converted some source from C to C++. Chrome has
checks against unannotated switch case fallthroughs in C++ code, so the
converted source needs annotations.

Bug: 990190
Change-Id: Ib92435b4877be936f837928a70b552ec4975d42a
Reviewed-on: https://chromium-review.googlesource.com/c/breakpad/breakpad/+/1898429
Reviewed-by: Mark Mentovai <mark@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>

src/common/macros.h

@@ -0,0 +1,45 @@
+// Copyright (c) 2019, Google Inc.
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef BREAKPAD_COMMON_MACROS_H_
+#define BREAKPAD_COMMON_MACROS_H_
+
+// Ensure that this macro definition stays in a private header file: clang
+// suggests the first macro expanding to [[clang::fallthrough]] in its
+// diagnostics, so if BP_FALLTHROUGH is visible in code depending on breakpad,
+// clang would suggest BP_FALLTHROUGH for code depending on breakpad, instead of
+// the client code's own fallthrough macro.
+// TODO(thakis): Once everyone uses C++17, use its [[fallthrough]] instead.
+#if defined(__clang__)
+#define BP_FALLTHROUGH [[clang::fallthrough]]
+#else
+#define BP_FALLTHROUGH
+#endif
+
+#endif // BREAKPAD_COMMON_MACROS_H_