[Ada] New routine to access file on command line

[Ada] New routine to access file on command line

This patch adds a new routine to query the first file argument of the
commandline without moving to the next file. This is needed in SPARK.

There is no impact on compilation.

2019-07-03  Johannes Kanig  <kanig@adacore.com>

gcc/ada/

	* osint.ads, osint.adb (Get_First_Main_File_Name): New routine
	to access the first file provided on the command line.

From-SVN: r272984