Add a function to merge trivia from a node into an existing trivia

Generally when a node is removed, we need to add its trivia to an
existing node. Add a small helper function to aid in this case.

Author: bnbarham

Sources/SwiftSyntax/Trivia.swift

@@ -25,40 +25,51 @@ public struct Trivia {
     self.pieces = Array(pieces)
   }
 
-  /// Creates Trivia with no pieces.
-  public static var zero: Trivia {
-    return Trivia(pieces: [])
-  }
-
   /// Whether the Trivia contains no pieces.
   public var isEmpty: Bool {
     pieces.isEmpty
   }
 
+  public var sourceLength: SourceLength {
+    return pieces.map({ $0.sourceLength }).reduce(.zero, +)
+  }
+
+  /// Get the byteSize of this trivia
+  public var byteSize: Int {
+    return sourceLength.utf8Length
+  }
+
   /// Creates a new `Trivia` by appending the provided `TriviaPiece` to the end.
   public func appending(_ piece: TriviaPiece) -> Trivia {
     var copy = pieces
     copy.append(piece)
     return Trivia(pieces: copy)
   }
 
-  public var sourceLength: SourceLength {
-    return pieces.map({ $0.sourceLength }).reduce(.zero, +)
+  /// Creates a new `Trivia` by appending the given trivia to the end.
+  public func appending(_ trivia: Trivia) -> Trivia {
+    var copy = pieces
+    copy.append(contentsOf: trivia.pieces)
+    return Trivia(pieces: copy)
   }
 
-  /// Get the byteSize of this trivia
-  public var byteSize: Int {
-    return sourceLength.utf8Length
+  /// Creates a new `Trivia` by appending the leading and trailing `Trivia`
+  /// of `triviaOf` to the end.
+  public func appending<T: SyntaxProtocol>(triviaOf node: T) -> Trivia {
+    var copy = pieces
+    copy.append(contentsOf: node.leadingTrivia.pieces)
+    copy.append(contentsOf: node.trailingTrivia.pieces)
+    return Trivia(pieces: copy)
   }
 
   /// Concatenates two collections of `Trivia` into one collection.
   public static func +(lhs: Trivia, rhs: Trivia) -> Trivia {
-    return Trivia(pieces: lhs.pieces + rhs.pieces)
+    return lhs.appending(rhs)
   }
 
   /// Concatenates two collections of `Trivia` into the left-hand side.
   public static func +=(lhs: inout Trivia, rhs: Trivia) {
-    lhs = lhs + rhs
+    lhs = lhs.appending(rhs)
   }
 }