Enable -Wimplicit-fallthrough.

Falling through switch cases on a clang build must now annotate the fallthrough
with the FALLTHROUGH_INTENDED macro.
Bug: 17731372

Change-Id: I836451cd5f96b01d1ababdbf9eef677fe8fa8324
diff --git a/build/Android.common_build.mk b/build/Android.common_build.mk
index 976a66e..d2d6d23 100644
--- a/build/Android.common_build.mk
+++ b/build/Android.common_build.mk
@@ -231,13 +231,20 @@
 ART_TARGET_CFLAGS += -DART_BASE_ADDRESS_MAX_DELTA=$(LIBART_IMG_TARGET_MAX_BASE_ADDRESS_DELTA)
 
 # Colorize clang compiler warnings.
+art_clang_cflags := -fcolor-diagnostics
+
+# Warn if switch fallthroughs aren't annotated.
+art_clang_cflags += -Wimplicit-fallthrough
+
 ifeq ($(ART_HOST_CLANG),true)
-  ART_HOST_CFLAGS += -fcolor-diagnostics
+  ART_HOST_CFLAGS += $(art_clang_cflags)
 endif
 ifeq ($(ART_TARGET_CLANG),true)
-  ART_TARGET_CFLAGS += -fcolor-diagnostics
+  ART_TARGET_CFLAGS += $(art_clang_cflags)
 endif
 
+art_clang_cflags :=
+
 ART_TARGET_LDFLAGS :=
 ifeq ($(TARGET_CPU_SMP),true)
   ART_TARGET_CFLAGS += -DANDROID_SMP=1