Add Vector{<:AbstractString} method for Cmd() (#49531)

Was annoyed that `Cmd` wouldn't take a vector containing SubStrings. So
I here add a constructor that converts an `AbstractString` vector to
`String`s.

base/cmd.jl

@@ -14,7 +14,7 @@ struct Cmd <: AbstractCmd
     env::Union{Vector{String},Nothing}
     dir::String
     cpus::Union{Nothing,Vector{UInt16}}
-    Cmd(exec::Vector{String}) =
+    Cmd(exec::Vector{<:AbstractString}) =
         new(exec, false, 0x00, nothing, "", nothing)
     Cmd(cmd::Cmd, ignorestatus, flags, env, dir, cpus = nothing) =
         new(cmd.exec, ignorestatus, flags, env,

test/spawn.jl

@@ -1032,3 +1032,10 @@ let effects = Base.infer_effects(x -> `a $x`, (Any,))
     @test !Core.Compiler.is_noub(effects)
     @test !Core.Compiler.is_consistent(effects)
 end
+
+# Test that Cmd accepts various AbstractStrings
+@testset "AbstractStrings" begin
+    args = split("-l /tmp")
+    @assert eltype(args) != String
+    @test Cmd(["ls", args...]) == `ls -l /tmp`
+end