DO NOT MERGE, Make default -Wimplicit-fallthrough as error.

Other branches might still have this warning for some build targets.

Bug: 112564944
Test: make checkbuild
Change-Id: I3db52b8a60bb28861fc9ba963a7d1cd1fe49e903
diff --git a/cc/config/clang.go b/cc/config/clang.go
index bbc78fa..74e8ae8 100644
--- a/cc/config/clang.go
+++ b/cc/config/clang.go
@@ -97,9 +97,8 @@
 	pctx.StaticVariable("ClangExtraCflags", strings.Join([]string{
 		"-D__compiler_offsetof=__builtin_offsetof",
 
-		// Make implicit fallthrough an error in the future.
+		// -Wimplicit fallthrough is not enabled by -Wall.
 		"-Wimplicit-fallthrough",
-		"-Wno-error=implicit-fallthrough",
 
 		// Help catch common 32/64-bit errors.
 		"-Werror=int-conversion",