GraphWriter: support the XDG open utility

This runs a suitable viewer on Unix desktop environments specified by
Freedesktop.org (GNOME, KDE, Linux distributions etc.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210147 91177308-0d34-0410-b5e6-96231b3b80d8
GraphWriter: try gv before xdg-open

Avoid changing behaviour for everyone who's used to the traditional ghostview
UI, especially since it knows how to stay in the foreground unlike xdg-open.

Amendment to r210147.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210148 91177308-0d34-0410-b5e6-96231b3b80d8