c++/v2.97 ice when inner template arg has default

[Larry Evans]

Larry Evans wrote:

> cd /home/evansl/prog_dev/gcc_all/tests/SubjStateType/
> make -k bugreport
> -------------------

...skipping

The problem is caused by the inner for loop in tsubst_template_parms not checking
for TYPE_CODE(default_value) == CONST_DECL.  Since tsubst calls tsubst_decl,
and tsubst_decl cannot handle CONST_DECL, the ICE is produced.

This may also solve 1088 since it sounds very similar.