[clang] [libcxx] [lldb] [clang]: implement std::meta::is_type (PR #211653)

[A. Jiang via cfe-commits]

frederick-vs-ja wrote:

> `__cpp_impl_reflection` is only enabled when `-freflection` is on

Why? Can we avoid `-freflection` and just enable reflection since C++26? Or can we make `-freflection` meaningless since C++26 and allow it in old modes as an extension (which is not very conforming, though)?

https://github.com/llvm/llvm-project/pull/211653