* decl.c (build_explicit_specifier): Don't check
	processing_template_decl.  Call instantiation_dependent_expression_p
	instead of value_dependent_expression_p.  Call
	instantiate_non_dependent_expr_sfinae before
	build_converted_constant_expr instead of calling
	instantiate_non_dependent_expr after it.  Add
	processing_template_decl_sentinel.

	* g++.dg/cpp2a/explicit14.C: New test.

From-SVN: r269131