clamp low voltage signal
There are OpAmp circuits called precision clamp (or precision rectifier, or precision diode).

 (source)

Regarding the long term reliability. (The 2nd question in the O.P.)
If you current-limit the input to the microcontroller with a resistor, you could apply -0.3V to the pin for a prolonged time without permanent damage.  Inportantly, the absolute maximum ratings are only about permanent damage, and not about performance.  You may or may not get good A/D performance.  Test it.