Fix clang test suite on Windows by reverting part of r347216

Otherwise, the clang analyzer tests fail on Windows when attempting to
unpickle AnalyzerTest objects in the worker processes. The pattern of,
add to path, import, remove from path, serialize, deserialize, doesn't
work. Once something gets added to the path, if we want to move it
across the wire for multiprocessing, we need to keep the module on
sys.path.

llvm-svn: 347254