[SourceKit] fix -Wqual-cast warning

When building SourceKit on Linux, this would result in a number of
-Wqual-cast warnings due to the `const` being dropped.  Add an explicit
cast.  NFC.

Author: compnerd

tools/SourceKit/include/SourceKit/Support/ThreadSafeRefCntPtr.h

@@ -120,7 +120,8 @@ class ThreadSafeRefCntPtr : ThreadSafeRefCntPtrImpl {
   }
 
   operator llvm::IntrusiveRefCntPtr<T>() const {
-    llvm::sys::ScopedLock L(*getMutex((void*)this));
+    llvm::sys::ScopedLock L(*getMutex(
+        reinterpret_cast<void *>(const_cast<ThreadSafeRefCntPtr *>(this))));
     llvm::IntrusiveRefCntPtr<T> Ref(Obj.load());
     return Ref;
   }