`@cfunction` does not throw `MethodError` / enforce argument types for single-dispatch targets

[topolarity]

We do not check that single-target @cfunction dispatches are fully-covered, which is required for to implement :call semantics properly:

julia> f(a::Cint,b::Cint) = a+b
julia> f(a::Float32, b::Cint) = a+b
julia> g(a::Float32, b::Cint) = a+b

julia> p = @cfunction(f, Any, (Any, Cint))
julia> q = @cfunction(g, Any, (Any, Cint))


julia> ccall(p, Any, (Any, Cint), Float32(1.52), 2)
3.52f0
julia> ccall(q, Any, (Any, Cint), Float32(1.52), 2)
3.52f0

julia> ccall(p, Any, (Any, Cint), 1.52, 2)
ERROR: MethodError: no method matching f(::Float64, ::Int32)
# ...
julia> ccall(q, Any, (Any, Cint), 1.52, 2)
2.0f0 # should be `MethodError` but instead UB

(and avoid UB)

found while trying to convince myself of the dispatch behavior in #62245

[vtjnash]

Throwing an error in cfunction is UB (though we should do it anyways)

[topolarity]

Yes, we should just panic if we decide this is UB to worry about (it's not clear to me that it's UB or just not thought through)