Build with -Wimplicit-fallthrough in Clang.

This will catch failures standalone before they reach consumers.

Change-Id: I1957df92e02ef7b4c609cd85420535c94ae3a04c
Reviewed-on: https://boringssl-review.googlesource.com/18005
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 7b7f934..cddffe2 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -51,7 +51,7 @@
   # https://clang.llvm.org/docs/DiagnosticsReference.html#wmissing-prototypes
   # https://clang.llvm.org/docs/DiagnosticsReference.html#wmissing-declarations
   if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wmissing-prototypes")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wmissing-prototypes -Wimplicit-fallthrough")
   endif()
 elseif(MSVC)
   set(MSVC_DISABLED_WARNINGS_LIST