A few minor path fixes for Windows.

When launching argdumper, there are a few problems with the
current logic.  First, on Windows, the file is called
argdumper.exe, not argdumper.  Second, Windows paths have
backslashes in them, and JSON treats <backslash><char> as an
escape sequence.  To fix the second problem, on Windows we
convert backslashes to forward slashes, since backslash isn't
a valid filename character anyway this shouldn't be a problem.

llvm-svn: 229784