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

[Aaron Ballman via cfe-commits]

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+/*===---- meta - header for reflection metafunctions -----------------------===
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AaronBallman wrote:

We did. The background here (as I recall it) is:

* libstdc++ has a `<meta>` header that only has declarations within it. The compiler is responsible for recognizing these meta functions and providing the actual implementation semantics.
* We had an allergic reaction to that design because we try hard not to recognize random STL interfaces from the compiler, we asked if they'd be willing to use builtins instead.
* They had an allergic reaction to adding a half a billion builtins and trivial function definitions when they can just recognize the library calls directly.
* So we decided that the only viable solution we have is for Clang to vend `<meta>`, otherwise there's no way for `-stdlib=libstdc++` to work in Clang because we couldn't consume their header. By vending it with Clang, our `<meta>` should be found first on the search path and work for any STL implementation.



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