[Ada] Alignment may be specified as zero

[Ada] Alignment may be specified as zero

An Alignment clause or an aspect_specification for Alignment may be
specified as 0, which is treated the same as 1.

2019-08-14  Bob Duff  <duff@adacore.com>

gcc/ada/

	* sem_ch13.adb (Get_Alignment_Value): Return 1 for Alignment 0,
	and do not give an error.
	* doc/gnat_rm/representation_clauses_and_pragmas.rst: Update the
	corresponding documentation.
	* gnat_rm.texi: Regenerate.

gcc/testsuite/

	* gnat.dg/alignment15.adb: New testcase.

From-SVN: r274473