Make it an error when LOCAL_CLANG is false.

Only projects in LOCAL_CLANG_EXCEPTION_PROJECTS can set LOCAL_CLANG to false.
Add external/gentoo/integration to this white list.

Bug: 30575506
Change-Id: Icd508686bc2eb8b9d400200166df80115af20da1
Test: build and find new warning in build.log
diff --git a/core/clang/config.mk b/core/clang/config.mk
index b31e419..7113892 100644
--- a/core/clang/config.mk
+++ b/core/clang/config.mk
@@ -149,6 +149,7 @@
   bionic/tests/ \
   device/huawei/angler/ \
   device/lge/bullhead/ \
+  external/gentoo/integration/ \
   external/valgrind/ \
   hardware/qcom/ \
   $(INTERNAL_LOCAL_CLANG_EXCEPTION_PROJECTS)