I need to know whether the radio receivers that are highly sensitive, [...] encounter less fading than the others with less sensitivity
No. Fading is mostly an effect of propagation rather than something that is happening within the radio. Also, it's more or less an umbrella term for a multitude of mechanisms that affect the signal strength on the way to the receiver, of which only some could be mitigated on the receiving side (like space diversity setups, more apt antennas for the planned propagation path, or sync detection against multipath distortion).
Sensitivity is always limited by noise, and the higher the noise floor the more a fading signal will be losing signal-to-noise ratio (SNR). On frequencies <10 MHz the natural noise (QRN) is usually way above the internal noise floor of almost any radio, therefore a sensitivity (or IOW noise floor) difference between 2 radios could only have impact on higher frequencies at night in an electrically very quiet environment, and the difference would be quite insignificant at such a low level.
Which specification of a radio receiver is responsible for mitigating fading?
That would be the Automatic Gain Control (AGC), the descriptive German word for that is "Schwundausgleich" meaning literally "fading compensation". What it does is automatically increasing gain when the signal fades and reducing gain when the signal is high. Since audio volume is linked to the signal strength in AM/SSB, it helps keeping the level constant.
