[Ada] Preliminary work to avoid full pathnames in ALI files

[Ada] Preliminary work to avoid full pathnames in ALI files

Normally, ALI files refer to source files using simple names. This allows files
to be moved around without disturbing things (causing extra recompilations,
etc). However, for configuration files, the full pathname is stored. This patch
preparates the code base to store the simple name in this case.

2018-05-29  Bob Duff  <duff@adacore.com>

gcc/ada/

	* lib-writ.adb (Write_ALI): Cleanup: avoid use of global var; call new
	To_Lower function.
	* libgnat/s-casuti.ads, libgnat/s-casuti.adb (To_Upper, To_Lower,
	To_Mixed): New functions.
	* osint.adb: Cleanup: use Is_Directory_Separator, which correctly
	allows both '\' and '/' on Windows.

From-SVN: r260860