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

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@316635 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld