If a function with a _Nullable parameter has at least
one caller, the checker behaves as though no other
parameter value is possible. This is because the static
analyzer doesn't re-analyse functions as top level
(with the exception of C++ copy and move operators, and
Fixed by modifying shouldSkipFunction to return false.
After this change, Clang-tidy even emits different
warnings for the same line of code, if appropriate:
"Nullable pointer is dereferenced" for a (simulated)
top-level call and "Dereference of null pointer"
for a call to the same function with a known null value.