LibUnicode: Avoid redundant UTF-8 validation in AK::String helpers

Userland/Libraries/LibUnicode/String.cpp

@@ -18,28 +18,28 @@ ErrorOr<String> String::to_lowercase(Optional<StringView> const& locale) const
 {
     StringBuilder builder;
     TRY(Unicode::Detail::build_lowercase_string(code_points(), builder, locale));
-    return builder.to_string();
+    return builder.to_string_without_validation();
 }
 
 ErrorOr<String> String::to_uppercase(Optional<StringView> const& locale) const
 {
     StringBuilder builder;
     TRY(Unicode::Detail::build_uppercase_string(code_points(), builder, locale));
-    return builder.to_string();
+    return builder.to_string_without_validation();
 }
 
 ErrorOr<String> String::to_titlecase(Optional<StringView> const& locale, TrailingCodePointTransformation trailing_code_point_transformation) const
 {
     StringBuilder builder;
     TRY(Unicode::Detail::build_titlecase_string(code_points(), builder, locale, trailing_code_point_transformation));
-    return builder.to_string();
+    return builder.to_string_without_validation();
 }
 
 ErrorOr<String> String::to_casefold() const
 {
     StringBuilder builder;
     TRY(Unicode::Detail::build_casefold_string(code_points(), builder));
-    return builder.to_string();
+    return builder.to_string_without_validation();
 }
 
 bool String::equals_ignoring_case(String const& other) const