[CFG Printer] Add support for writing the dot files with a custom

prefix.

Use this to direct these files to a specific location in the test suite
so that we don't write files out to random directories (or fail if the
working directory isn't writable).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@344014 91177308-0d34-0410-b5e6-96231b3b80d8