nested types: GenerateHeaderIncludes()

Visitor-based function gathers all include files for a header with
nested types.

However GenerateHeader() doesn't support nested types yet. Upcoming
changes will make it to generate class decl for each nested type
recursively.

Bug: 182508839
Test: aidl_unittests
Test: tests/golden_test.sh check
Change-Id: I2bb6b160a6a99fef7576c4c22f10e6015f3dc5c0
diff --git a/aidl_to_cpp.h b/aidl_to_cpp.h
index f6974f5..620897f 100644
--- a/aidl_to_cpp.h
+++ b/aidl_to_cpp.h
@@ -61,7 +61,7 @@
 void AddHeaders(const AidlTypeSpecifier& type, const AidlTypenames& typenames,
                 std::set<std::string>* headers);
 
-void AddHeaders(const AidlDefinedType& parcelable, std::set<std::string>* headers);
+std::string CppHeaderForType(const AidlDefinedType& defined_type);
 }  // namespace cpp
 }  // namespace aidl
 }  // namespace android