Get rid of shorthand if let syntax to fix compilation with Swift 5.6

Author: evnik

Sources/SwiftSyntax/Syntax.swift

@@ -529,10 +529,10 @@ public extension SyntaxProtocol {
     if let token = Syntax(self).as(TokenSyntax.self) {
       target.write(String(describing: token.tokenKind))
       if includeTrivia {
-        if let leadingTrivia, !leadingTrivia.isEmpty {
+        if let leadingTrivia = leadingTrivia, !leadingTrivia.isEmpty {
           target.write(" leadingTrivia=\(leadingTrivia.debugDescription)")
         }
-        if let trailingTrivia, !trailingTrivia.isEmpty {
+        if let trailingTrivia = trailingTrivia, !trailingTrivia.isEmpty {
           target.write(" trailingTrivia=\(trailingTrivia.debugDescription)")
         }
       }

Sources/SwiftSyntax/Trivia.swift.gyb

@@ -150,7 +150,7 @@ public struct Trivia {
 
 extension Trivia: CustomDebugStringConvertible {
    public var debugDescription: String {
-     if count == 1, let first {
+     if count == 1, let first = first {
        return first.debugDescription
      }
      return "[" + map(\.debugDescription).joined(separator: ", ") + "]"

Sources/SwiftSyntax/gyb_generated/Trivia.swift

@@ -276,7 +276,7 @@ public struct Trivia {
 
 extension Trivia: CustomDebugStringConvertible {
    public var debugDescription: String {
-     if count == 1, let first {
+     if count == 1, let first = first {
        return first.debugDescription
      }
      return "[" + map(\.debugDescription).joined(separator: ", ") + "]"