Add LOCAL_CLANG variable to switch C/C++ build environment to Clang.

Change-Id: I8ca1c24f51aa5546ac9618d31566a52e4fdb1436

Conflicts:

	core/clear_vars.mk
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 63f44b3..a7d9509 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -118,6 +118,8 @@
 LOCAL_MODULE_OWNER:=
 LOCAL_CTS_TEST_PACKAGE:=
 LOCAL_CTS_TEST_RUNNER:=
+LOCAL_CLANG:=
+LOCAL_COMPILER_DEPENDENCIES:=
 
 # Trim MAKEFILE_LIST so that $(call my-dir) doesn't need to
 # iterate over thousands of entries every time.