c++: structural_comptypes addition

c++: structural_comptypes addition

I had to debug structural_comptypes, and its complex if conditions and
tail calling of same_type_p made that hard.  I'd hope we can turn the
eqivalent of return boolean_fn () ? true : false; into a tail call of
the boolean.  We also were not dealing with TYPEOF_TYPE.

	gcc/cp/
	* typeck.c (structural_comptypes): [DECLTYPE_TYPE] break
	apart complex if.
	[UNDERLYING_TYPE]: Use an if.
	[TYPEOF_TYPE]: New.