[lit] Respect LLVM_LIT_TOOLS_DIR when looking for 'tar' on Windows.

Changes in D38977 expect 'tar' to be found in one of PATH directories.
On Windows, one might opt to use LLVM_LIT_TOOLS_DIR rather than add MSYS
tools directory to the PATH. In that case, tests for lld failed on run.

Differential Revision: https://reviews.llvm.org/D39278

llvm-svn: 316635