[sourcekit] Merge SourceKit into the Swift repo.

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.

tools/SourceKit/lib/Support/ThreadSafeRefCntPtr.cpp

@@ -0,0 +1,12 @@
+#include "SourceKit/Support/ThreadSafeRefCntPtr.h"
+#include <functional>
+
+using namespace SourceKit;
+using llvm::sys::Mutex;
+
+static const size_t MtxCount = 16;
+static Mutex Mtxs[MtxCount];
+
+Mutex *ThreadSafeRefCntPtrImpl::getMutex(void *Ptr) {
+  return Mtxs + (std::hash<const void*>()(Ptr) & (MtxCount-1));
+}