[Statepoints] Make operator bool() explicit.

(Addressing post-commit review.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241288 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/Statepoint.h

@@ -86,7 +86,7 @@ public:
     CallArgsBeginPos = 5,
   };
 
-  operator bool() const {
+  explicit operator bool() const {
     // We do not assign non-statepoint CallSites to StatepointCS.
     return (bool)StatepointCS;
   }