Add += operator for Trivia

Author: fwcd

Sources/SwiftSyntax/gyb_generated/Trivia.swift

@@ -341,6 +341,11 @@ public func +(lhs: Trivia, rhs: Trivia) -> Trivia {
   return Trivia(pieces: lhs.pieces + rhs.pieces)
 }
 
+/// Concatenates two collections of `Trivia` into the left-hand side.
+public func +=(lhs: inout Trivia, rhs: Trivia) {
+  lhs = lhs + rhs
+}
+
 extension TriviaPiece: Equatable {}
 
 extension TriviaPiece {

Tests/SwiftSyntaxBuilderTest/TriviaTests.swift

@@ -24,4 +24,12 @@ final class TriviaTests: XCTestCase {
       /* An inline comment */ static var test: String
       """)
   }
+
+  func testTriviaConcatenation() {
+    let x = Trivia.newline
+    let y = x
+    y += .space
+    XCTAssertEqual(y, x + .space)
+    XCTAssertEqual(y, [.newline, .space])
+  }
 }