[Ada] Missing diagnostic of categorization dependency

[Ada] Missing diagnostic of categorization dependency

This patch modifies the analysis of subprogram bodies to catch a case where a
pure subprogram body unit depends on non-pure units.

2018-01-11  Hristian Kirtchev  <kirtchev@adacore.com>

gcc/ada/

	* sem_ch6.adb (Analyze_Subprogram_Body_Helper): Check the
	categorization of a subprogram body which does not complete a previous
	declaration.

gcc/testsuite/

	* gnat.dg/pure_subp_body.adb, gnat.dg/pure_subp_body_pkg.ads: New
	testcase.

From-SVN: r256516