I noticed that tsubst of a TEMPLATE_ID_EXPR was losing the location
information from its EXPR_LOCATION.  Then I noticed that
cxx_eval_constant_expression was also throwing away location information for
variable references.

	* pt.c (tsubst_copy_and_build) [TEMPLATE_ID_EXPR]: Remember the
	location of a variable template-id.
	* constexpr.c (cxx_eval_constant_expression): Get expr location
	before stripping location wrappers.
	(non_const_var_error): Take location argument.

From-SVN: r278755