<regex>: Follow the Standard in regex_traits::transform()

[Arzaghi]

Fixes #1004

[StephanTLavavej]

Thanks for fixing this bug! I pushed two microscopic changes:

• The Standard depicts str but we need to use ugly _Str.
• Purely stylistic: although the Standard depicts .length() being called, we conventionally always call the synonymous size() for basic_string. I also changed one pre-existing occurrence nearby. Now, the only length() calls in the entire STL are for sub_match.

On my machine, the libcxx test is auto-skipped as unsupported due to lacking LOCALE_cs_CZ_ISO8859_2, but I manually verified that (without those lines) the test fails to compile before your change, and successfully compiles after your change. (We might need to revise skipped_tests.txt when we mirror this to the MSVC-internal repo, not sure, but I'll take care of that if necessary.)

[StephanTLavavej]

Thanks for fixing this user-reported bug and removing another entry from our list of skipped libcxx tests! 🪲 😸