Enable -Wimplicit-fallthrough for clang as well as GCC

All instances of this warning should already be fixed across all LLVM
subprojects, at least on Linux.

llvm-svn: 345887

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -580,6 +580,7 @@ if (LLVM_ENABLE_WARNINGS AND (LLVM_COMPILER_IS_GCC_COMPATIBLE OR CLANG_CL))
     append("-Wno-long-long" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
   endif()
 
+  add_flag_if_supported("-Wimplicit-fallthrough" IMPLICIT_FALLTHROUGH_FLAG)
   add_flag_if_supported("-Wcovered-switch-default" COVERED_SWITCH_DEFAULT_FLAG)
   append_if(USE_NO_UNINITIALIZED "-Wno-uninitialized" CMAKE_CXX_FLAGS)
   append_if(USE_NO_MAYBE_UNINITIALIZED "-Wno-maybe-uninitialized" CMAKE_CXX_FLAGS)