[clang] [clang] Implement a bitwise_copyable builtin type trait. (PR #86512)

[A. Jiang via cfe-commits]

frederick-vs-ja wrote:

IIUC this intrinsic is unrelated to assignment operators. Do you have any plan for `__is_bitwise_assignable`, which should determine whether the assignment operator equivalently copies the value representation, possibly modulo the vptrs?

```C++
struct Cat {}; // bitwise copy assignable
struct Leopard : Cat { // is_trivially_assignable_v returns true but not bitwise copy assignable
    int spots_;
    Leopard& operator=(Leopard&) = delete;
    using Cat::operator=; // We can detect this by metaprogramming as operator= overloads have wrong types.
};
struct LeopardHouse { // is_trivially_assignable_v returns true but not bitwise copy assignable
    Leopard _;
    // Non-bitwise-assignability *can't* be detected by metaprogramming since operator= overloads have right types.
};
```

https://github.com/llvm/llvm-project/pull/86512