c++: cp_stabilize_reference and non-dep exprs [PR111919]

After the removal of NON_DEPENDENT_EXPR, cp_stabilize_reference (which
used to just exit early for NON_DEPENDENT_EXPR) is now more prone to
passing a weird templated tree to middle-end routines, which for the
testcase below leads to a crash from contains_placeholder_p.  It seems
the best fix is to just exit early when in a template context, like we
do in the closely related function cp_save_expr.

	PR c++/111919

gcc/cp/ChangeLog:

	* tree.cc (cp_stabilize_reference): Do nothing when
	processing_template_decl.

gcc/testsuite/ChangeLog:

	* g++.dg/template/non-dependent27.C: New test.