Add default -Wimplicit-fallthrough.

After we fix all existing such warnings, it can be changed to an error.

Bug: 112564944
Test: make checkbuild
Change-Id: I61ee3a24a3f992ef018cfe6c995e957416551183

cc/config/clang.go

@@ -103,6 +103,10 @@ func init() {
 	pctx.StaticVariable("ClangExtraCflags", strings.Join([]string{
 		"-D__compiler_offsetof=__builtin_offsetof",
 
+		// Make implicit fallthrough an error in the future.
+		"-Wimplicit-fallthrough",
+		"-Wno-error=implicit-fallthrough",
+
 		// Help catch common 32/64-bit errors.
 		"-Werror=int-conversion",