Unicode: assume foldable ccall in category_code (#54394)
Following on from https://github.com/JuliaLang/julia/pull/54346, this
marks the `ccall` in `category_code` as foldable. This lets us compute
the results of several functions at compile time, such as:
```julia
julia> @code_typed (() -> isletter('C'))()
CodeInfo(
1 ─ return true
) => Bool
julia> @code_typed (() -> isnumeric('C'))()
CodeInfo(
1 ─ return false
) => Bool
julia> @code_typed (() -> ispunct('C'))()
CodeInfo(
1 ─ return false
) => Bool
```
