[clang][analyzer][NFC] Remove unnecessary FALLTHROUGH markers

[gribozavr]

Summary

They are redundant with the [[fallthrough]]; attribute that follows.

clang/lib/CodeGen/CGCall.cpp

case ABIArgInfo::CoerceAndExpand: {
         Address eltAddr = Builder.CreateStructGEP(addr, i);
         llvm::Value *elt = CI;
         if (requiresExtract)
           elt = Builder.CreateExtractValue(elt, unpaddedIndex++);
         else
           assert(unpaddedIndex == 0);
         Builder.CreateStore(elt, eltAddr);
       }
-      // FALLTHROUGH
       [[fallthrough]];
     }
 
     case ABIArgInfo::InAlloca:
     case ABIArgInfo::Indirect: {
       RValue ret = convertTempToRValue(SRetPtr, RetTy, SourceLocation());
       if (UnusedReturnSizePtr)
         PopCleanupBlock();

clang/lib/StaticAnalyzer/Core/ExprEngine.cpp

case Stmt::CXXOperatorCallExprClass: {
             Pred = Bldr.generateNode(OCE, Pred, NewState, /*tag=*/nullptr,
                                      ProgramPoint::PreStmtKind);
             // Did we cache out?
             if (!Pred)
               break;
           }
         }
       }
-      // FALLTHROUGH
       [[fallthrough]];
     }
 
     case Stmt::CallExprClass:
     case Stmt::CXXMemberCallExprClass:
     case Stmt::UserDefinedLiteralClass:
       Bldr.takeNodes(Pred);
       VisitCallExpr(cast<CallExpr>(S), Pred, Dst);

clang/lib/StaticAnalyzer/Core/SValBuilder.cpp

case Stmt::ImplicitCastExprClass: {
     case CK_BitCast: {
       const Expr *SE = CE->getSubExpr();
       std::optional<SVal> Val = getConstantVal(SE);
       if (!Val)
         return std::nullopt;
       return evalCast(*Val, CE->getType(), SE->getType());
     }
     }
-    // FALLTHROUGH
     [[fallthrough]];
   }
 
   // If we don't have a special case, fall back to the AST's constant evaluator.
   default: {
     // Don't try to come up with a value for materialized temporaries.
     if (E->isGLValue())
       return std::nullopt;