Apparently the Backend_Layout target configuration parameter was renamed to
Frontend_Layout a long time ago (and their meanings are opposite). However,
some comments were still referencing the no longer existing Backend_Layout.
This patch fixes such references.

No test provided, because only comments has been modified.

2018-05-24  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* layout.ads, repinfo.ads: Fix references to renamed Backend_Layout
	configuration parameter.

From-SVN: r260642