Adjust includes of moved Clang headers

Per 0d150db214e2aa13a825b563c7238e1243d61db1 (llvm-project).

tools/SourceKit/lib/Support/ImmutableTextBuffer.cpp

@@ -11,13 +11,13 @@
 //===----------------------------------------------------------------------===//
 
 #include "SourceKit/Support/ImmutableTextBuffer.h"
-#include "clang/Rewrite/Core/RewriteRope.h"
+#include "llvm/ADT/RewriteRope.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/SourceMgr.h"
 
 using namespace SourceKit;
 using namespace llvm;
-using clang::RewriteRope;
+using llvm::RewriteRope;
 
 void ImmutableTextUpdate::anchor() {}