[Support] On Windows, generate PDF files for graphs and open with associated viewer

Summary: Windows system rarely have good PostScript viewers installed, but PDF viewers are common. So for viewing graphs, generate PDF files and open with the associated PDF viewer using cmd.exe's start command.

Reviewers: Bigcheese, aaron.ballman

Subscribers: aaron.ballman, JakeVanAdrighem, dwiberg, llvm-commits

Differential Revision: http://reviews.llvm.org/D11877

llvm-svn: 245290