Xcode seems to think that [some] builtins can be used from Swift. It
autocompletes popcount for example, and command-clicking the function opens
a bona fide interface file, with the following declaration (among others):
public func __builtin_popcount(_: UInt32) -> Int32
However, when I try to compile code that uses that function, I get an
unresolved identifier compiler error.
I guess I need to import some module, but which one?
Generally speaking, how to find in which module some symbol is defined?