c++: ICE with nested anonymous union [PR117153]

c++: ICE with nested anonymous union [PR117153]

In a template, for

  union {
    union {
      T d;
    };
  };

build_anon_union_vars crates a malformed COMPONENT_REF: we have no
DECL_NAME for the nested anon union so we create something like "object.".
Most of the front end doesn't seem to care, but if such a tree gets to
potential_constant_expression, it can cause a crash.

We can use FIELD directly for the COMPONENT_REF's member.  tsubst_stmt
should build up a proper one in:

    if (VAR_P (decl) && !DECL_NAME (decl)
	&& ANON_AGGR_TYPE_P (TREE_TYPE (decl)))
      /* Anonymous aggregates are a special case.  */
      finish_anon_union (decl);

	PR c++/117153

gcc/cp/ChangeLog:

	* decl2.cc (build_anon_union_vars): Use FIELD for the second operand
	of a COMPONENT_REF.

gcc/testsuite/ChangeLog:

	* g++.dg/other/anon-union6.C: New test.
	* g++.dg/other/anon-union7.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>