Add diagnostic message for identifier editor placeholder

Author: kimdv

Sources/SwiftParser/Parser.swift

@@ -412,7 +412,12 @@ extension Parser {
     keywordRecovery: Bool = false
   ) -> (RawUnexpectedNodesSyntax?, RawTokenSyntax) {
     if allowIdentifierLikeKeywords {
-      if let (_, handle) = self.canRecoverTo(anyIn: IdentifierTokens.self) {
+        if currentToken.isEditorPlaceholder {
+            return (
+                RawUnexpectedNodesSyntax(elements: [RawSyntax(self.consumeAnyToken())], arena: self.arena),
+                self.missingToken(.identifier, text: nil)
+            )
+        } else if let (_, handle) = self.canRecoverTo(anyIn: IdentifierTokens.self) {
         return self.eat(handle)
       }
     } else {

Tests/SwiftParserTest/translated/IdentifiersTests.swift

@@ -83,9 +83,10 @@ final class IdentifiersTests: XCTestCase {
     AssertParse(
       """
       // Placeholders are recognized as identifiers but with error.
-      func <#some name#>() {}
+      func 1️⃣<#some name#>() {}
       """,
       diagnostics: [
+        DiagnosticSpec(message: "editor placeholder in source file")
         // TODO: (good first issue) Old parser expected error on line 2: editor placeholder in source file
       ]
     )