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

This is a reland of c7c5b492

Changes since revert:
- Save position instead of using PushBack
- Allow private-name-surrogate-pair to fail on no_i18n builds

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: I8a92953549f5b38bfa004488db42bf9d835e1222
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3368361Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78513}

src/parsing/scanner.cc

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

test/mjsunit/mjsunit.status

@@ -449,6 +449,9 @@
 
   # Temporal intl tests won't work in no_i18n
   'temporal/function-exist': [FAIL],
+
+  # Non-BMP characters currently aren't considered identifiers in no_i18n
+  'harmony/private-name-surrogate-pair': [PASS,FAIL],
 }],  # 'no_i18n'
 
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