c++: Document why TARGET_EXPR is not handled in tsubst_expr

c++: Document why TARGET_EXPR is not handled in tsubst_expr

TARGET_EXPR represents semantic temporary objects and is deliberately
not handled by tsubst routines, which expect syntactic templated trees.
Add a comment and gcc_unreachable to make this explicit.

gcc/cp/ChangeLog:

	* pt.cc (tsubst_expr): Add TARGET_EXPR case with explanatory
	comment and gcc_unreachable.

Signed-off-by: Egas Ribeiro <egas.g.ribeiro@tecnico.ulisboa.pt>
Reviewed-by: Patrick Palka <ppalka@redhat.com>