Backpor from mainline

Backpor from mainline
	2018-06-02  Eric Botcazou  <ebotcazou@adacore.com>

	* gcc-interface/decl.c (gnat_to_gnu_entity) <E_Constant>: If this is
	not a definition, retrieve the expression only if it's a compile-time
	known value if we are just annotating types.

	* gcc-interface/utils.c (convert): Do not try to upcast properly for a
	conversion between tagged types in type_annotate_only mode.

From-SVN: r261482