What affect does charger wattage have on lifetime of LiIon batteries?
The battery concerned is LiIon or LiPo.
This is relevant to the question.
I'll say LiIon below - it applies equally to LiPo.

Short answer: Charging at somewhat below max allowed rate may cause a modest improvement in battery cycle life but it will not be a major factor. 

Longer:

A LiIon battery has a max allowed charge rate that is specified by the battery maker.
This is usually the C/1 rate = a current in mA equal to the mAh capacity of the battery.
IF you charged at the C/1 rate for one hour the mAh transferred would equal the mAh nominally available from a fully charged battery.

However, this is not how LiIon chemistry works.
From flat you charge at Imax or less (usually C/1 or less) until V_charge_max is reached (typically 4.2V per series cell) and THEn the charger holds Vcharge at V_charge_max and the battery accepts a declining current with time that depends on battery chemistry. When I charge falls to I_charge_min charging is complete and Vcharge MUST be removed. Floating a LiIon battery at V_charge_max tends to destroy it rapidly.

In your case the 85W charger is probably sized to charge at I_Charge_max when the charging circuitry allows it to. The 45W charger max current is below I_charge_max, so until V_charge_max is reached charging occurs at I_best_45W_supply_can_manage. 

This reduced charge rate MAY increase battery lifetime but probably not appreciably as while many other factors are discussed and documented this is not. You cannot charge the battery "too fast" by using a higher wattage power supply as the actual charger circuitry is internal to the laptop and it will limit Ichg to C/1 or whatever rate the battery is rated at.

Lower Ichg reduces heating, probably allows somewhat more even deposition of Lithium in the battery and is probably therefore less mechanically wearing on the battery. mA major life affecting factor of LiIon batteries is that they are mechanically "pumped" by the charge/discharge action. The batteries beat themselves to death. [This is a major factor in the long life of LiFePO4 batteries - an internal Olivine structure provides mechanical support for the battery structure - and takes up room so reduces battery capacity.]