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
diff --git a/tools/warn.py b/tools/warn.py
index f17e8ac..f118263 100755
--- a/tools/warn.py
+++ b/tools/warn.py
@@ -74,6 +74,9 @@
         'description':'make: overriding commands/ignoring old commands',
         'patterns':[r".*: warning: overriding commands for target .+",
                     r".*: warning: ignoring old commands for target .+"] },
+    { 'category':'make',    'severity':severity.HIGH,   'members':[], 'option':'',
+        'description':'make: LOCAL_CLANG is false',
+        'patterns':[r".*: warning: LOCAL_CLANG is set to false"] },
     { 'category':'C/C++',   'severity':severity.HIGH,     'members':[], 'option':'-Wimplicit-function-declaration',
         'description':'Implicit function declaration',
         'patterns':[r".*: warning: implicit declaration of function .+",