perf: eliminate spurious string copies in cpp2rust converter

[Copilot]

Several hot-path functions in the converter were performing redundant or avoidable std::string allocations during type-to-string conversion and struct/record processing.

Changes

• mapper.cpp — ToString(): Added std::move() when passing local strings to normalizeTranslationRule() (by-value param). Since raw_string_ostream is unbuffered, the string is fully populated before the call; move avoids a copy on every type serialization.

• converter.cpp — recordDerivesCopy(): Mapper::Map(f->getType()) was called twice per field for two separate starts_with() checks. Result is now cached in a local.

• converter.cpp — GetDefaultAsString(): Mapper::ToString(qual_type) was called twice in successive else if branches. Result is now cached before the checks.

• converter.cpp — EmitRustStruct() / VisitCXXRecordDecl(): Replaced multi-step temporaries:

  // Before: constructs 2-3 temporary std::string objects
  std::string(keyword::kImpl) + ' ' + struct_name
  
  // After: single allocation
  std::format("{} {}", keyword::kImpl, struct_name)

• converter_lib.cpp — GetFileName(): Eliminated an intermediate file_name_as_string copy by constructing std::filesystem::path directly from StringRef iterators and using file_path.string() as the fallback return value.