c++: Add TARGET_EXPR comments

c++: Add TARGET_EXPR comments

Discussing the 98469 patch and class prvalues with Jakub led me to
double-check our handling of TARGET_EXPR in constexpr.c, and add a note
about why we don't strip them in parameter initialization.  And another to
clarify that we're handling an INIT_EXPR in a place we do strip them.

gcc/cp/ChangeLog:

	* constexpr.c (cxx_bind_parameters_in_call): Add comment.
	(cxx_eval_store_expression): Add comment.