[clang] [Clang] Introduce __builtin_is_modifiable_lvalue() (PR #132524)

[A. Jiang via cfe-commits]

frederick-vs-ja wrote:

> It looks like this is similar to `__builtin_constant_p` - what is the proposed behavior wrt. side effects in the evaluated expression? gcc and clang disagree about this a lot currently: https://godbolt.org/z/rbneznT9z

IIUC this intrinsic shouldn't be similar to `__builtin_constant_p`. It's just investing properties of the type and value category of an expression (like `std::is_lvalue_reference_v<decltype((expr))>` in C++ but with more magics).

https://github.com/llvm/llvm-project/pull/132524