Refine the SlangRS::compile() API.

This makes compile() methods in SlangRS more reasonable. And this is
part of commits for ODR. The SlangRS::compile() now compiles "a group of
RS files" instead of one by one individually.
diff --git a/Android.mk b/Android.mk
index 4986a45..05066e5 100644
--- a/Android.mk
+++ b/Android.mk
@@ -191,6 +191,7 @@
 	DeclNodes.inc    \
 	DiagnosticCommonKinds.inc   \
 	DiagnosticDriverKinds.inc	\
+	DiagnosticFrontendKinds.inc	\
 	DiagnosticSemaKinds.inc	\
 	StmtNodes.inc	\
 	RSCCOptions.inc