[build_script.py] Allow lit path to be specified
[build_script.py] Allow lit path to be specified

Allow the path to the lit tool to be specified to the build script.
This will allow the correct lit executable to be used when invoked from
the apple/swift build script.

Also, address SR-981 by surfacing a clear, actionable error message
when the lit executable cannot be found.

tiagomartinho/swift-corelibs-xctest,apple/swift-corelibs-xctest,tiagomartinho/swift-corelibs-xctest,yoonapps/swift-corelibs-xctest,apple/swift-corelibs-xctest,yoonapps/swift-corelibs-xctest