Fix indentation when there is a leading new line

Author: kimdv

Sources/SwiftSyntax/Syntax.swift

@@ -355,7 +355,23 @@ public extension SyntaxProtocol {
   /// Returns a new syntax node with its leading trivia replaced
   /// by the provided trivia.
   func withLeadingTrivia(_ leadingTrivia: Trivia) -> Self {
-    return Self(Syntax(data.withLeadingTrivia(leadingTrivia)))!
+    var leadingTrivia: [TriviaPiece] = leadingTrivia.pieces
+
+    // If there is 3 or more, then we see if the first two is a newline and a space
+    // If so, we need to replace add the correct amount of space after each newline
+    // comming after to have the right indentation of the code.
+    if leadingTrivia.count > 2,
+       case .newlines = leadingTrivia[0],
+       case .spaces(let spaces) = leadingTrivia[1] {
+      
+      for i in (2..<leadingTrivia.count).reversed() {
+        if case .newlines = leadingTrivia[i] {
+          leadingTrivia.insert(.spaces(spaces), at: i + 1)
+        }
+      }
+    }
+
+    return Self(Syntax(data.withLeadingTrivia(Trivia(pieces: leadingTrivia))))!
   }
 
   /// Returns a new syntax node with its trailing trivia replaced