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
2 files changed