c++: Fix offsetof use [PR 98232]

[Nathan Sidwell]

offsetof is underspecified.  GCC happened to accept an unneeded
explicit scoping, clang does not.

        gcc/cp/
        * module.cc (dumper::push): Clangify offsetof use.

pushed
--
Nathan Sidwell

diff --git i/gcc/cp/module.cc w/gcc/cp/module.cc
index d2806966e43..e9ea18636b4 100644
--- i/gcc/cp/module.cc
+++ w/gcc/cp/module.cc
@@ -4153,7 +4153,7 @@ dumper::push (module_state *m)
       /* Create or extend the dump implementor.  */
       unsigned current = dumps ? dumps->stack.length () : 0;
       unsigned count = current ? current * 2 : EXPERIMENT (1, 20);
-      size_t alloc = (offsetof (impl, impl::stack)
+      size_t alloc = (offsetof (impl, stack)
 		      + impl::stack_t::embedded_size (count));
       dumps = XRESIZEVAR (impl, dumps, alloc);
       dumps->stack.embedded_init (count, current);