[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.

unittests/SourceKit/Support/ImmutableTextBufferTest.cpp

@@ -0,0 +1,33 @@
+#include "SourceKit/Support/ImmutableTextBuffer.h"
+#include "gtest/gtest.h"
+
+using namespace SourceKit;
+using namespace llvm;
+
+TEST(EditableTextBuffer, Updates) {
+  const char *Text = "hello world";
+
+  EditableTextBufferManager BufMgr;
+  EditableTextBufferRef EdBuf = BufMgr.getOrCreateBuffer("/a/test", Text);
+  ImmutableTextBufferRef Buf = EdBuf->getBuffer();
+
+  EXPECT_EQ(Buf->getText(), Text);
+
+  Buf = EdBuf->insert(6, "all ")->getBuffer();
+  EXPECT_EQ(Buf->getText(), "hello all world");
+
+  Buf = EdBuf->erase(9, 6)->getBuffer();
+  EXPECT_EQ(Buf->getText(), "hello all");
+
+  Buf = EdBuf->replace(0, 5, "yo")->getBuffer();
+  EXPECT_EQ(Buf->getText(), "yo all");
+
+  EdBuf = BufMgr.resetBuffer("/a/test", Text);
+  EdBuf->insert(6, "all ");
+  EdBuf->erase(9, 6);
+  EdBuf->replace(0, 5, "yo");
+  Buf = EdBuf->getSnapshot()->getBuffer();
+  EXPECT_EQ(Buf->getText(), "yo all");
+
+  EXPECT_EQ(Buf->getFilename(), "/a/test");
+}