[PATCH 1/2] Introduce __builtin_is_virtual_base_of

[Giuseppe D'Angelo]

Hi,

The attached patch is a stab at adding the necessary compiler builtin to 
support std::is_virtual_base_of (P2985R0, approved for C++26). The name 
of the builtin matches the one just merged into clang:

https://github.com/llvm/llvm-project/issues/98310

The next patch will add the libstdc++ bits.
-- 
Giuseppe D'Angelo
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