Revert "[scanner] Combine surrogate pairs at start when scanning private names"

This reverts commit c7c5b492.

Reason for revert: Looks like test needs to be disabled for noi18n: https://ci.chromium.org/ui/p/v8/builders/ci/V8%20Linux%20-%20noi18n%20-%20debug/40832/overview

Original change's description:
> [scanner] Combine surrogate pairs at start when scanning private names
>
> Bug: v8:12523
> Change-Id: Ic3779fe6f20965d177d99d0a570a735df72e4fde
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3366994
> Reviewed-by: Leszek Swirski <leszeks@chromium.org>
> Commit-Queue: Shu-yu Guo <syg@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#78493}

Bug: v8:12523
Change-Id: I678d69a7acb793ed03ce049a05c37685d0cdee1a
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3368106
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#78495}

src/parsing/scanner.cc

@@ -497,17 +497,15 @@ Token::Value Scanner::ScanPrivateName() {
   next().literal_chars.Start();
   DCHECK_EQ(c0_, '#');
   DCHECK(!IsIdentifierStart(kEndOfInput));
-  Advance();
-  if (IsIdentifierStart(c0_) ||
-      (CombineSurrogatePair() && IsIdentifierStart(c0_))) {
-    AddLiteralChar('#');
-    Token::Value token = ScanIdentifierOrKeywordInner();
-    return token == Token::ILLEGAL ? Token::ILLEGAL : Token::PRIVATE_NAME;
+  if (!IsIdentifierStart(Peek())) {
+    ReportScannerError(source_pos(),
+                       MessageTemplate::kInvalidOrUnexpectedToken);
+    return Token::ILLEGAL;
   }
 
-  PushBack('#');  // Undo Advance()
-  ReportScannerError(source_pos(), MessageTemplate::kInvalidOrUnexpectedToken);
-  return Token::ILLEGAL;
+  AddLiteralCharAdvance();
+  Token::Value token = ScanIdentifierOrKeywordInner();
+  return token == Token::ILLEGAL ? Token::ILLEGAL : Token::PRIVATE_NAME;
 }
 
 Token::Value Scanner::ScanTemplateSpan() {

test/mjsunit/harmony/private-name-surrogate-pair.js

-// Copyright 2022 the V8 project authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-class C1 {
-  #𖥸 = 42;
-  m() { return this.#𖥸; }
-}
-
-assertEquals((new C1).m(), 42);
-
-class C2 {
-  #𖥸() { return 42; }
-  m() { return this.#𖥸(); }
-}
-
-assertEquals((new C2).m(), 42);