Yet another backend for hidl-gen, this one generates a C-compatible header file

containing those enum types annotated in the package like so:

@export
enum Foo {
    ...
};

Optionally, the name to be used for the type declaration in the header file
can be different from that used in the .hal interface description by specifying

@export(name="foo_t")
enum Foo {
    ...
};

Finally, overriding the name to be empty, i.e.

@export(name="")
enum Foo {
    ...
};

will cause the generator to emit an anonymous enum.

Bug: 31800672
Change-Id: Idffb2c1700af1c7fd312941d80c3373add8ae558
Test: make
diff --git a/Type.cpp b/Type.cpp
index 54f5956..5c65a3e 100644
--- a/Type.cpp
+++ b/Type.cpp
@@ -390,6 +390,14 @@
     CHECK(!"Should not be here");
 }
 
+void Type::appendToExportedTypesVector(
+        std::vector<const Type *> * /* exportedTypes */) const {
+}
+
+status_t Type::emitExportedHeader(Formatter & /* out */) const {
+    return OK;
+}
+
 ////////////////////////////////////////
 
 TemplatedType::TemplatedType() : mElementType(nullptr) {