skip unnecessary alias-check in collect(::AbstractArray) from copyto! (#55748)

As discussed on Slack with @MasonProtter & @jakobnissen, `collect`
currently does a usually cheap - but sometimes expensive - aliasing
check (via `unalias`->`mightalias`->`dataid` -> `objectid`) before
copying contents over; this check is unnecessary, however, since the
source array is newly created and cannot possibly alias the input.
This PR fixes that by swapping from `copyto!` to `copyto_unaliased!` in
the `_collect_indices` implementations where the swap is straightforward
(e.g., it is not so straightforward for the fallback
`_collect_indices(indsA, A)`, so I skipped it there).

This improves the following example substantially:
```jl
struct GarbageVector{N} <: AbstractVector{Int}
    v :: Vector{Int}
    garbage :: NTuple{N, Int}
end
GarbageVector{N}(v::Vector{Int}) where N = GarbageVector{N}(v, ntuple(identity, Val(N)))
Base.getindex(gv::GarbageVector, i::Int) = gv.v[i]
Base.size(gv::GarbageVector) = size(gv.v)

using BenchmarkTools
v = rand(Int, 10)
gv = GarbageVector{100}(v)
@Btime collect($v);  # 30 ns (v1.10.4)  -> 30 ns (PR)
@Btime collect($gv); # 179 ns (v1.10.4) -> 30 ns (PR)
```

Relatedly, it seems the fact that `mightalias` is comparing immutable
contents as well - and hence slowing down the `unalias` check for the
above `GarbageVector` via a slow `objectid` on tuples - is suboptimal. I
don't know how to fix that though, so I'd like to leave that outside
this PR. (Probably related to
#26237)

Co-authored-by: Matt Bauman <[email protected]>

Author: thchr

base/array.jl

@@ -748,9 +748,16 @@ function _collect(cont, itr, ::HasEltype, isz::SizeUnknown)
     return a
 end
 
-_collect_indices(::Tuple{}, A) = copyto!(Array{eltype(A),0}(undef), A)
-_collect_indices(indsA::Tuple{Vararg{OneTo}}, A) =
-    copyto!(Array{eltype(A)}(undef, length.(indsA)), A)
+function _collect_indices(::Tuple{}, A)
+    dest = Array{eltype(A),0}(undef)
+    isempty(A) && return dest
+    return copyto_unaliased!(IndexStyle(dest), dest, IndexStyle(A), A)
+end
+function _collect_indices(indsA::Tuple{Vararg{OneTo}}, A)
+    dest = Array{eltype(A)}(undef, length.(indsA))
+    isempty(A) && return dest
+    return copyto_unaliased!(IndexStyle(dest), dest, IndexStyle(A), A)
+end
 function _collect_indices(indsA, A)
     B = Array{eltype(A)}(undef, length.(indsA))
     copyto!(B, CartesianIndices(axes(B)), A, CartesianIndices(indsA))