Fix GIMPLE verification failure in LTO mode on Ada code

Fix GIMPLE verification failure in LTO mode on Ada code

The issue is that tree_is_indexable doesn't return the same result for
a FIELD_DECL with QUAL_UNION_TYPE and the QUAL_UNION_TYPE, resulting
in two instances of the QUAL_UNION_TYPE in the bytecode.  The result
for the type is the correct one (false, since it is variably modified)
while the result for the field is falsely true because:

  else if (TREE_CODE (t) == FIELD_DECL
           && lto_variably_modified_type_p (DECL_CONTEXT (t)))
    return false;

is not satisfied.  The reason for this is that the DECL_QUALIFIER of
fields of a QUAL_UNION_TYPE depends on a discriminant in Ada, which
means that the size of the type does too (CONTAINS_PLACEHOLDER_P),
which in turn means that it is reset to a mere PLACEHOLDER_EXPR by
free_lang_data, which finally means that the size of DECL_CONTEXT is
too, so RETURN_TRUE_IF_VAR is false.

In other words, the CONTAINS_PLACEHOLDER_P property of the DECL_QUALIFIER
of fields of a QUAL_UNION_TYPE hides the variably_modified_type_p property
of  these fields, if you look from the outside.

	PR middle-end/93961
	* tree.c (variably_modified_type_p) <RECORD_TYPE>: Recurse into
	fields whose type is a qualified union.