LibXML: Iterate over code points in Parser::parse_internal()

`s_restricted_characters` contains code points, not UTF-8 bytes.

Previously, LibXML errored out on the string '中文' since it contains
the byte 0x96, which is in the restricted char set. But this only
happened on ABIs where char is signed as this UTF-8 byte had the most
significant bit set, therefore the comparison was never true for
bytes >= 0x80.

Since `Utf8CodePointIterator::index()` doesn't exist, the byte offset
is now calculated using `it.ptr() - view.bytes()`.

This makes LibXML/TestParser pass on non-x86.

Userland/Libraries/LibXML/Parser/Parser.cpp

@@ -221,9 +221,10 @@ ErrorOr<void, ParseError> Parser::parse_internal()
     }
 
     auto matched_source = m_source.substring_view(0, m_lexer.tell());
-    if (auto it = find_if(matched_source.begin(), matched_source.end(), s_restricted_characters); !it.is_end()) {
+    Utf8View view { matched_source };
+    if (auto it = find_if(view.begin(), view.end(), s_restricted_characters); it != view.end()) {
         return parse_error(
-            m_lexer.position_for(it.index()),
+            m_lexer.position_for(it.ptr() - view.bytes()),
             ByteString::formatted("Invalid character #{:x} used in document", *it));
     }