Fix log1p with complex Irrational argument

[giordano]

Even if you don't like Complex{Irrational}, this change is a slight performance improvement, because after PR #21219 float(T) doesn't allocate with bignumbers and Rationals

[musm]

Unrelated but isn't it strange that float has multiple meanings whether it acts on Types or Data?
float(2) = 2.0
float(Int) = Float64
Is this a common thing for methods to have multiple meanings depending on whether the arguments are Types or Data values

[giordano]

It looks like not everybody like this (see, e.g., #21218 (comment)), but this is useful to define more efficient methods, like in this case. In addition, you often want to determine the floating type corresponding to T, the alternative is to use the fallback definition of float, that is typeof(float(zero(T))), but it may not be possible to define zero(T) for all T (like Irrational).

[tkelman]

lgtm