Exceeding the max allowed ratings always lead to unpredictable results, so better avoid that. You may use an external schottki diode to cut the undershot.
As Bob mentioned, we can not say how the device behaves in case of exceeding the spec. range described in datasheet.
You must follow "DC input voltage on GPIO" described in datasheet to avoid an unforeseen issue...
And you have to care externally (from device), e.g. using schottki diode as Bob suggested.
I agree with Bob and TakashiM, it is best to avoid inputs over-specification. in your case, a Schottky diode should work.
However, you might also look at place a series resistor between the input circuit and the input of the PSoC. The most likely problem that occurs when the input voltage goes below GND (as in your case) or above VDD is something called input latch-up current. It has to do with the FET on outputs and ESD protection diodes on inputs can be destroyed if the forward current exceeds the latch-up current. If this happens, it's common for it to appear as permanent a short-circuit to VSS or VDD.
For most 5V input circuits, the latch-up current is anywhere from +/-10mA to +/-200mA. By placing a schottky diode at the pin for inputs, you should be able to 'stub' most excessive voltages. Placing a series resistor will also limit the latch-up current. The value of the resistor should be as high as your design can tolerate without effecting the application intent. For both digital and analog input circuits the value depends on the frequency of changes you expect to support. If the frequency of the input is higher, a lower value may be needed. (eq. 1MHz => 100 ohms.) As the input frequency lowers, you can probably tolerate a higher resistance value such as 10K. To determine which value works best you can perform some empirical studies or you can calculate the RC equivalent value for your circuit.
The diagram below shows the ESD diode configuration in the PSoC . It also shows the schottky diode protection for VSS and VDD along with the latch-up current limit resistor. In your case, I recommend the resistor and the VSS protection schottky at a minimum.