[astgen] Fix argument/param labels/names.

Author: zoecarver

lib/ASTGen/Sources/ASTGen/Decls.swift

@@ -91,7 +91,9 @@ extension ASTGenVisitor {
     let secondName: UnsafeMutableRawPointer?
     let type: UnsafeMutableRawPointer?
 
-    if let nodeFirstName = node.firstName {
+    if let nodeFirstName = node.firstName,
+       // Swift AST represnts "_" as nil.
+       nodeFirstName.text != "_" {
       var text = nodeFirstName.text
       firstName = text.withUTF8 { buf in
         SwiftASTContext_getIdentifier(ctx, buf.baseAddress, buf.count)

lib/ASTGen/Sources/ASTGen/Exprs.swift

@@ -64,8 +64,10 @@ extension ASTGenVisitor {
 
   public func visit(_ node: TupleExprElementListSyntax) -> ASTNode {
     let elements = node.map { self.visit($0).rawValue }
-    let labels: [BridgedIdentifier] = node.enumerated().map { (idx, e) in
-      var name = e.label?.text ?? "\(idx)"
+    let labels: [BridgedIdentifier?] = node.map {
+      guard var name = $0.label?.text else {
+        return nil
+      }
       return name.withUTF8 { buf in
         SwiftASTContext_getIdentifier(ctx, buf.baseAddress, buf.count)
       }