improve allunique's type stability (#56161)

Caught by aviatesk/JET.jl#667.

Author: aviatesk

base/set.jl

@@ -549,8 +549,8 @@ function allunique(A::StridedArray)
     if length(A) < 32
         _indexed_allunique(A)
     elseif OrderStyle(eltype(A)) === Ordered()
-        a1, rest1 = Iterators.peel(A)
-        a2, rest = Iterators.peel(rest1)
+        a1, rest1 = Iterators.peel(A)::Tuple{Any,Any}
+        a2, rest = Iterators.peel(rest1)::Tuple{Any,Any}
         if !isequal(a1, a2)
             compare = isless(a1, a2) ? isless : (a,b) -> isless(b,a)
             for a in rest

test/sets.jl

@@ -644,6 +644,7 @@ end
     @test !allunique((NaN, NaN))
     # Known length 1, need not evaluate:
     @test allunique(error(x) for x in [1])
+    # @test_opt allunique(Int[])
 end
 
 @testset "allunique(f, xs)" begin