which should give better error on ambiguous method

[LilithHafner]

Calling an ambiguous method gives a nice error message. Calling which on it does not:

julia> f(a::Int, b) = a+b
f (generic function with 1 method)

julia> f(a, b::Int) = a-b
f (generic function with 2 methods)

julia> f(2,2) # very helpful error message
ERROR: MethodError: f(::Int64, ::Int64) is ambiguous.

Candidates:
  f(a::Int64, b)
    @ Main REPL[21]:1
  f(a, b::Int64)
    @ Main REPL[22]:1

Possible fix, define
  f(::Int64, ::Int64)

Stacktrace:
 [1] top-level scope
   @ REPL[23]:1

julia> which(f, Tuple{Int,Int}) # less helpful error message
ERROR: no unique matching method found for the specified argument types
Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:35
 [2] _which
   @ ./reflection.jl:1491 [inlined]
 [3] _which
   @ ./reflection.jl:1489 [inlined]
 [4] which(tt::Any)
   @ Base ./reflection.jl:1517
 [5] which(f::Any, t::Any)
   @ Base ./reflection.jl:1508
 [6] top-level scope
   @ REPL[24]:1

From #22988.