tests: Add an 'llvm-lit' tool (script), which gets generated as part of the

tests: Add an 'llvm-lit' tool (script), which gets generated as part of the
build and has the object build directory baked into it. This allows 'llvm-lit'
to properly find the information needed to run the test suite in all cases,
without requiring the user to have LLVM or 'lit' available in their PATH, for
example.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@110000 91177308-0d34-0410-b5e6-96231b3b80d8