[clangd] Fix bool conversion operator of UniqueFunction

Usages of it were giving compiler errors because of the missing
explicit conversion.

llvm-svn: 320591

clang-tools-extra/clangd/Function.h

@@ -49,7 +49,7 @@ public:
                     FunctionCallImpl<typename std::decay<Callable>::type>>(
             std::forward<Callable>(Func))) {}
 
-  operator bool() { return CallablePtr; }
+  operator bool() { return bool(CallablePtr); }
 
   Ret operator()(Args... As) {
     assert(CallablePtr);