SourceKit doesn't use them and if any unrecognised LLVM options are
passed to llvm::cl::ParseCommandLineOptions() it calls exit(), bringing
down SourceKit.
Also use fprintf instead of llvm::errs() in Logging.cpp as it uses a
global C++ object that had already been destructed when logging the
above failure.
Resolves rdar://problem/38314383
The code goes into its own sub-tree under 'tools' but tests go under 'test',
so that running 'check-swift' will also run all the SourceKit tests.
SourceKit is disabled on non-darwin platforms.
