fix: do not use abi::__forced_unwind with libc++, even with gcc instead of clang (#2569)

* add support for LLVM compiler

* Update include/pybind11/pybind11.h

__forced_unwind libc++ vs libstdc++

Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>

Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>

include/pybind11/pybind11.h

@@ -741,7 +741,7 @@ protected:
         } catch (error_already_set &e) {
             e.restore();
             return nullptr;
-#if defined(__GNUG__) && !defined(__clang__)
+#if defined(__GNUG__) && !defined(_LIBCPP_VERSION)
         } catch ( abi::__forced_unwind& ) {
             throw;
 #endif