c++: Fix up handling of references to anon union members in initializers [PR53932]

c++: Fix up handling of references to anon union members in initializers [PR53932]

For anonymous union members we create artificial VAR_DECLs which
have DECL_VALUE_EXPR for the actual COMPONENT_REF.  That works
just fine inside of functions (including global dynamic constructors),
because during gimplification such VAR_DECLs are gimplified as
their DECL_VALUE_EXPR.  This is also done during regimplification.

But references to these artificial vars in DECL_INITIAL expressions
aren't ever replaced by the DECL_VALUE_EXPRs, so we end up either
with link failures like on the testcase below, or worse ICEs with
LTO.

The following patch fixes those during cp_fully_fold_init where we
already walk all the trees (!data->genericize means that
function rather than cp_fold_function).

2023-01-19  Jakub Jelinek  <jakub@redhat.com>

	PR c++/53932
	* cp-gimplify.c (cp_fold_r): During cp_fully_fold_init replace
	DECL_ANON_UNION_VAR_P VAR_DECLs with their corresponding
	DECL_VALUE_EXPR.

	* g++.dg/init/pr53932.C: New test.

(cherry picked from commit 9b9a989adc042b304572fd6d4ade46b47be6ccb8)