[SourceKit] Add missing std::atomic include

`UIdent.h` references `std::atomic`, but does not include
`<atomic>`. This results in an error when compiling on Linux.

Author: modocache

tools/SourceKit/include/SourceKit/Support/UIdent.h

@@ -15,6 +15,7 @@
 
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Compiler.h"
+#include <atomic>
 
 namespace llvm {
   class raw_ostream;