[Lit] Fix to prevent creation of "%SystemDrive%" directory on Windows.

This patch propogates the environment variable SYSTEMDRIVE on Windows when
running the unit tests. This prevents the creation of a directory named
"%SystemDrive%" when running the unit tests from FileSystemTest that use the
function llvm::sys::fs::remove_directories which in turn uses SHFileOperationW.
It is within SHFileOperationW that this environment variable may be used and if
undefined causes the creation of a "%SystemDrive%" directory in the current
directory.

Differential Revision: https://reviews.llvm.org/D32910

llvm-svn: 302409