fix: --checkapi works with enum constants

--checkapi should compare const values with "literal" values, not
with evaluated values because --checkapi task can't resolve imported
types.

--checkapi=equal was relying on "Dump()" which relies on type/value/
resolution.

Now --checkapi=equal dumps const values using literals, not evaluated
values.

Bug: 179950161
Test: m aidl_unittests
Change-Id: I62c84f060036260bfcd9c1020e32715993378e26
diff --git a/aidl_checkapi.cpp b/aidl_checkapi.cpp
index f07e7e6..de5c9ad 100644
--- a/aidl_checkapi.cpp
+++ b/aidl_checkapi.cpp
@@ -41,10 +41,18 @@
 using std::string;
 using std::vector;
 
+struct DumpForEqualityVisitor : DumpVisitor {
+  DumpForEqualityVisitor(CodeWriter& out) : DumpVisitor(out) {}
+
+  void DumpConstantValue(const AidlTypeSpecifier&, const AidlConstantValue& c) {
+    out << c.Literal();
+  }
+};
+
 static std::string Dump(const AidlDefinedType& type) {
   string code;
   CodeWriterPtr out = CodeWriter::ForString(&code);
-  DumpVisitor visitor(*out);
+  DumpForEqualityVisitor visitor(*out);
   type.DispatchVisit(visitor);
   out->Close();
   return code;