[Ada] Fix assertion failure on pragma Compile_Time_Error in generic unit

[Ada] Fix assertion failure on pragma Compile_Time_Error in generic unit

There is no point in validating 'Alignment or 'Size of an entity
declared in a generic unit after the back-end has been run, since such
an entity is not passed to the back-end, and this can even lead to an
assertion failure.

2018-11-14  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* sem_prag.adb (Process_Compile_Time_Warning_Or_Error): Don't
	register a compile-time warning or error for 'Alignment or 'Size
	of an entity declared in a generic unit.

gcc/testsuite/

	* gnat.dg/compile_time_error1.adb,
	gnat.dg/compile_time_error1.ads,
	gnat.dg/compile_time_error1_pkg.ads: New testcase.

From-SVN: r266127