fir.call side effects are hard to describe in a useful way using
`MemoryEffectOpInterface` because it is impossible to list which memory
location a user procedure read/write without doing a data flow analysis
of its body (even PURE procedures may read from any module variable,
Fortran SIMPLE procedure from F2023 will allow that, but they are far
from common at that point).
Fortran language specifications allow the compiler to deduce
that a procedure call cannot access a variable in many cases
This patch leverages this to extend `fir::AliasAnalysis::getModRef` to
deal with fir.call.
This will allow implementing "array = array_function()" optimization in
a future patch.
