[analyzer][NFC] Simplify BugType handling in core.BitwiseShift (#74609)

Eliminate the `mutable unique_ptr` hack because it's no longer needed.
(This cleanup could be done anywhere, I'm doing it here now because it
was me who published this checker with the old hack when it was already
superfluous.)

Author: NagyDonat

clang/lib/StaticAnalyzer/Checkers/BitwiseShiftChecker.cpp

@@ -344,7 +344,7 @@ BitwiseShiftValidator::createBugReport(StringRef ShortMsg, StringRef Msg) const
 } // anonymous namespace
 
 class BitwiseShiftChecker : public Checker<check::PreStmt<BinaryOperator>> {
-  mutable std::unique_ptr<BugType> BTPtr;
+  BugType BT{this, "Bitwise shift", "Suspicious operation"};
 
 public:
   void checkPreStmt(const BinaryOperator *B, CheckerContext &Ctx) const {
@@ -353,11 +353,7 @@ class BitwiseShiftChecker : public Checker<check::PreStmt<BinaryOperator>> {
     if (Op != BO_Shl && Op != BO_Shr)
       return;
 
-    if (!BTPtr)
-      BTPtr = std::make_unique<BugType>(this, "Bitwise shift",
-                                        "Suspicious operation");
-
-    BitwiseShiftValidator(B, Ctx, *BTPtr, Pedantic).run();
+    BitwiseShiftValidator(B, Ctx, BT, Pedantic).run();
   }
 
   bool Pedantic = false;