Bug 575893 - [performance] improve file search: file sort

Avoid file.getLocation() during sort: only calculate location once.

Change-Id: Ic0953c430385c74d4b879347ac1699f38c9099d5
Signed-off-by: Joerg Kubitz <jkubitz-eclipse@gmx.de>
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.text/+/185194
Tested-by: Platform Bot <platform-bot@eclipse.org>
Reviewed-by: Mickael Istria <mistria@redhat.com>