Make code compile for Swift 5.5

Author: DougGregor

lib/ASTGen/Sources/ASTGen/Diagnostics.swift

@@ -124,10 +124,10 @@ func emitDiagnostic(
 }
 
 extension SourceManager {
-  private func diagnoseSingle(
+  private func diagnoseSingle<Node: SyntaxProtocol>(
     message: String,
     severity: DiagnosticSeverity,
-    node: some SyntaxProtocol,
+    node: Node,
     position: AbsolutePosition,
     highlights: [Syntax] = [],
     fixItChanges: [FixIt.Change] = []

lib/ASTGen/Sources/ASTGen/SourceManager.swift

@@ -136,7 +136,7 @@ extension SourceManager {
 
     var detached = context.detach(node)
 
-    if let operatorTable {
+    if let operatorTable = operatorTable {
       detached = operatorTable.foldAll(node) { _ in }.as(Node.self)!
     }