Fixed the test suite on MacOSX so that...

Fixed the test suite on MacOSX so that "test/api/multithreaded/TestMultithreaded.py" works without errors.

The problem was that the @skipIfNoSBHeaders on darwin was trying to use self.lib_dir when it hadn't been set yet.

I looked at the code and places were required to set "self.lib_dir" for no real reason as all places that used it just used the LLDB_LIB_DIR environment variable. So I removed all uses of self.lib_dir and replaced them to use 'os.environ["LLDB_LIB_DIR"]'. Did the same for self.implib_dir.

llvm-svn: 251315