[Ada] Enable using absolute paths in -fdiagnostics-format=json output

This commit makes GNAT use absolute paths in -fdiagnostics-format=json's
output when -gnatef is present on the command line. This makes life
easier for tools that ingest GNAT's output.

gcc/ada/

	* doc/gnat_ugn/building_executable_programs_with_gnat.rst:
	Document new behavior.
	* errout.adb (Write_JSON_Location): Output absolute paths when
	needed.
	* switch-c.adb (Scan_Front_End_Switches): Update -gnatef
	comment.
	* usage.adb (Usage): Update description of -gnatef.
	* gnat_ugn.texi: Regenerate.