Use QuickSort rather than MergeSort by default for Union{<:Number, Missing} arrays

[nalimilan]

This eliminates allocations and is about 30% faster for Vector{Union{Float64,Missing}}.

Mitigates (but doesn't completely fix) #27781.

Benchmark added at JuliaCI/BaseBenchmarks.jl#211.

[nalimilan]

@nanosoldier runbenchmarks("sort" || "union", vs = ":master")

[nanosoldier]

Your benchmark job has completed - possible performance regressions were detected. A full report can be found here. cc @ararslan

[nalimilan]

Really nice results (apart from a few unrelated things):

ID	time ratio	memory ratio
["union", "array", "("sort", BigFloat, true)"]	1.01 (15%)	0.66 (1%)
["union", "array", "("sort", BigInt, true)"]	0.86 (15%)	0.66 (1%)
["union", "array", "("sort", Bool, true)"]	0.63 (15%)	0.65 (1%)
["union", "array", "("sort", Float32, true)"]	0.85 (15%)	0.66 (1%)
["union", "array", "("sort", Float64, true)"]	0.84 (15%)	0.66 (1%)
["union", "array", "("sort", Int64, true)"]	0.68 (15%)	0.66 (1%)
["union", "array", "("sort", Int8, true)"]	0.66 (15%)	0.65 (1%)

[nalimilan]

OK to merge?