CPlusPlus: Handle C++20 concepts in parser

Change-Id: I8c6b8b1ba3f36b83cd1d667bec9830271147b1ac
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/libs/3rdparty/cplusplus/ASTfwd.h

@@ -146,6 +146,7 @@ class OperatorAST;
 class OperatorFunctionIdAST;
 class ParameterDeclarationAST;
 class ParameterDeclarationClauseAST;
+class PlaceholderTypeSpecifierAST;
 class PointerAST;
 class PointerLiteralAST;
 class PointerToMemberAST;
@@ -166,6 +167,7 @@ class QtPropertyDeclarationItemAST;
 class QualifiedNameAST;
 class RangeBasedForStatementAST;
 class ReferenceAST;
+class RequiresExpressionAST;
 class ReturnStatementAST;
 class SimpleDeclarationAST;
 class SimpleNameAST;
@@ -178,6 +180,7 @@ class StdAttributeSpecifierAST;
 class StringLiteralAST;
 class SwitchStatementAST;
 class TemplateDeclarationAST;
+class ConceptDeclarationAST;
 class TemplateIdAST;
 class TemplateTypeParameterAST;
 class ThisExpressionAST;
@@ -185,6 +188,7 @@ class ThrowExpressionAST;
 class TrailingReturnTypeAST;
 class TranslationUnitAST;
 class TryBlockStatementAST;
+class TypeConstraintAST;
 class TypeConstructorCallAST;
 class TypeIdAST;
 class TypeidExpressionAST;