[Support] Reapply r245289 "Always wait for GraphViz before opening the viewer"

The change was accidentally undone by r245290.

Original log message:
When calling DisplayGraph and a PS viewer is chosen, two programs are executed: The GraphViz generator and the PostScript viewer. Always wait for the generator to finish to ensure that the .ps file is written before opening the viewer for that file. DisplayGraph's wait parameter refers to whether to wait until the user closes the viewer.

This happened on Windows and if none of the options to open the .dot file directly applies, also on Linux.

Differential Revision: http://reviews.llvm.org/D11876

llvm-svn: 247980