[SwiftSyntaxBuilder] Add support for control flow statements in result builders

Add the `buildOptional` etc. result builder constructs to add support for control-flow constructs inside the result builders.

Author: ahoppen

Sources/SwiftSyntaxBuilder/ResultBuilders.swift.gyb

@@ -26,8 +26,65 @@ import SwiftSyntax
 @resultBuilder
 public struct ${node.syntax_kind}Builder {
 %     element_type = syntax_buildable_child_type(node.collection_element_type, node.collection_element, node.is_token())
-  public static func buildBlock(_ elements: ${element_type}...) -> ${node.syntax_kind} {
-    ${node.syntax_kind}(elements)
+  
+  /// The type of individual statement expressions in the transformed function,
+  /// which defaults to Component if buildExpression() is not provided.
+  public typealias Expression = ${element_type}
+
+  /// The type of a partial result, which will be carried through all of the
+  /// build methods.
+  public typealias Component = [${element_type}]
+
+  /// The type of the final returned result, which defaults to Component if
+  /// buildFinalResult() is not provided.
+  public typealias FinalResult = ${node.syntax_kind}
+
+  /// Required by every result builder to build combined results from
+  /// statement blocks.
+  public static func buildBlock(_ components: Component...) -> Component {
+    components.flatMap { $0 }
+  }
+
+  /// If declared, provides contextual type information for statement
+  /// expressions to translate them into partial results.
+  public static func buildExpression(_ expression: Expression) -> Component {
+    [expression]
+  }
+
+  /// Enables support for `if` statements that do not have an `else`.
+  public static func buildOptional(_ component: Component?) -> Component {
+    component ?? []
+  }
+
+  /// With buildEither(second:), enables support for 'if-else' and 'switch'
+  /// statements by folding conditional results into a single result.
+  public static func buildEither(first component: Component) -> Component {
+    component
+  }
+
+  /// With buildEither(first:), enables support for 'if-else' and 'switch'
+  /// statements by folding conditional results into a single result.
+  public static func buildEither(second component: Component) -> Component {
+    component
+  }
+
+  /// Enables support for 'for..in' loops by combining the
+  /// results of all iterations into a single result.
+  public static func buildArray(_ components: [Component]) -> Component {
+    components.flatMap { $0 }
+  }
+
+  /// If declared, this will be called on the partial result of an 'if
+  /// #available' block to allow the result builder to erase type
+  /// information.
+  public static func buildLimitedAvailability(_ component: Component) -> Component {
+    component
+  }
+
+  /// If declared, this will be called on the partial result from the outermost
+  /// block statement to produce the final returned result.
+  public static func buildFinalResult(_ component: Component) -> FinalResult {
+    .init(component)
   }
 }
 
@@ -36,4 +93,4 @@ extension ${node.syntax_kind} {
 }
 
 %   end
-% end
+% end