Allow no discard attribute

Author: arturjpv

UnrealAngelscriptParser/Grammar/UnrealAngelscriptLexer.g4

@@ -128,6 +128,8 @@ Inherited: 'inherited';
 
 Namespace: 'namespace';
 
+NoDiscard: 'no_discard';
+
 Nullptr: 'nullptr';
 
 Operator: 'operator';

UnrealAngelscriptParser/Grammar/UnrealAngelscriptParser.g4

@@ -432,7 +432,7 @@ declaratorDef:
 	declaratorid | declaratorDef (parametersAndQualifiers | LeftBracket constantExpression? RightBracket);
 
 parametersAndQualifiers:
-	LeftParen parameterDeclarationClause? RightParen thisModifier? refqualifier?;
+	LeftParen parameterDeclarationClause? RightParen thisModifier? refqualifier? NoDiscard?;
 
 thisModifier: Const | AcceptTemporaryThis;