[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.