[GraphWriter] Attempt to open .dot files with xdg-open/open first

[GraphWriter] Attempt to open .dot files with xdg-open/open first

Most desktop environments let the users specify his preferred application per
file type. On mac/linux we can use open/xdg-open for that and should try this
first before starting a heuristic search for various programs.

Differential Revision: http://reviews.llvm.org/D6534

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234031 91177308-0d34-0410-b5e6-96231b3b80d8