Replace Maybe with std::optional

With c++17, std::optional provides the functionality that Maybe
provided.

Bug: 183215655
Test: aapt2_tests
Change-Id: I62bb9c2fa4985dc5217a6ed153df92b85ad9a034
diff --git a/tools/aapt2/link/ReferenceLinker.h b/tools/aapt2/link/ReferenceLinker.h
index 770f1e5..b460853 100644
--- a/tools/aapt2/link/ReferenceLinker.h
+++ b/tools/aapt2/link/ReferenceLinker.h
@@ -97,11 +97,11 @@
 
   // Resolves the attribute reference and returns an xml::AaptAttribute if successful.
   // If resolution fails, outError holds the error message.
-  static Maybe<xml::AaptAttribute> CompileXmlAttribute(const Reference& reference,
-                                                       const CallSite& callsite,
-                                                       IAaptContext* context,
-                                                       SymbolTable* symbols,
-                                                       std::string* out_error);
+  static std::optional<xml::AaptAttribute> CompileXmlAttribute(const Reference& reference,
+                                                               const CallSite& callsite,
+                                                               IAaptContext* context,
+                                                               SymbolTable* symbols,
+                                                               std::string* out_error);
 
   // Writes the resource name to the DiagMessage, using the
   // "orig_name (aka <transformed_name>)" syntax.