`func_extended` ZIR instructions now have a one of the unused flags used
as a `has_comptime_bits` boolean. When set, it means 1 or more
parameters are `comptime`. In this case, there is a u32 per every 32
parameters (usually just 1 u32) with each bit indicating whether the
corresponding parameter is `comptime`.

Sema uses this information to correctly mark generic functions as
generic. There is now a TODO compile error in place in case a generic
function call happens. A future commit will do the generic function call
implementation.