Issue warning on projects with LOCAL_CLANG set to false.

* LOCAL_CLANG_EXCEPTION_PROJECTS is the list of project prefixes.
* New warning message is detected by warn.py as high severity.

Bug: 30575506
Change-Id: I87a38b2ba6bab1d2e5e7a9d5ec9557f0e609727c
Test: build and find new warning in build.log
3 files changed