Type-assert `DataType.layout` in some Base functions (#59886)
This prevents a bunch of invalidations, specifically these ones in Julia
1.12 with UnsafePointers.jl (CC @cjdoris):
```julia
inserting convert(P::Type{<:Ptr}, p::UnsafePointers.UnsafePtr) @ UnsafePointers ~/git/UnsafePointers.jl/src/UnsafePointers.jl:66 invalidated:
mt_backedges: 1: signature Tuple{typeof(convert), Type{Ptr{Base.DataTypeLayout}}, Any} triggered MethodInstance for Base.datatype_npointers(::Type{T} where T<:(NamedTuple{names, T} where {T<:Tuple, names})) (4 children)
2: signature Tuple{typeof(convert), Type{Ptr{Base.DataTypeLayout}}, Any} triggered MethodInstance for Base.datatype_npointers(::Type{T} where T<:AbstractString) (4 children)
3: signature Tuple{typeof(convert), Type{Ptr{Base.DataTypeLayout}}, Any} triggered MethodInstance for Base.datatype_npointers(::Type{T} where T<:Tuple{Symbol, Any}) (4 children)
4: signature Tuple{typeof(convert), Type{Ptr{Base.DataTypeLayout}}, Any} triggered MethodInstance for Base.datatype_npointers(::Type{T} where T<:(NamedTuple{(:message, :position, :hint), _A} where _A)) (4 children)
5: signature Tuple{typeof(convert), Type{Ptr{Base.DataTypeLayout}}, Any} triggered MethodInstance for Base.datatype_npointers(::Type{T} where T<:(SubString{_A} where _A)) (4 children)
6: signature Tuple{typeof(convert), Type{Ptr{Base.DataTypeLayout}}, Any} triggered MethodInstance for Base.datatype_alignment(::Type{T} where T<:(NamedTuple{(:message, :position, :hint), _A} where _A)) (15 children)
7: signature Tuple{typeof(convert), Type{Ptr{Base.DataTypeLayout}}, Any} triggered MethodInstance for Base.datatype_alignment(::Type{T} where T<:(SubString{_A} where _A)) (19 children)
8: signature Tuple{typeof(convert), Type{Ptr{Base.DataTypeLayout}}, Any} triggered MethodInstance for Base.datatype_alignment(::Type{T} where T<:Tuple{Any, Symbol, Symbol}) (21 children)
9: signature Tuple{typeof(convert), Type{Ptr{Base.DataTypeLayout}}, Any} triggered MethodInstance for Base.datatype_alignment(::Type{T} where T<:Tuple{Symbol, Any}) (23 children)
10: signature Tuple{typeof(convert), Type{Ptr{Base.DataTypeLayout}}, Any} triggered MethodInstance for Base.datatype_alignment(::Type{T} where T<:AbstractString) (52 children)
11: signature Tuple{typeof(convert), Type{Ptr{Base.DataTypeLayout}}, Any} triggered MethodInstance for Base.datatype_alignment(::Type{T} where T<:(NamedTuple{names, T} where {T<:Tuple, names})) (1739 children)
```
Using Cthulhu to look at the last invalidation with 1739 children showed
this:
```julia
datatype_alignment(dt::DataType) @ Base ~/.julia/juliaup/julia-1.12.1+0.x64.linux.gnu/share/julia/base/runtime_internals.jl:544
544 function datatype_alignment(dt::Type{T} where T<:(NamedTuple{names, T} where {T<:Tuple, names})::DataType)::Any
545 @_foldable_meta
546 (dt::Type{T} where T<:(NamedTuple{names, T} where {T<:Tuple, names}).layout::Any == C_NULL::Core.Const(Ptr{Nothing}(0x0000000000000000)))::Any && throw(UndefRefError()::Core.Const(UndefRefError()))
547 alignment::Any = unsafe_load(convert(Ptr{DataTypeLayout}::Type{Ptr{Base.DataTypeLayout}}, dt::Type{T} where T<:(NamedTuple{names, T} where {T<:Tuple, names}).layout::Any)::Any)::Any.alignment::Any
548 return Int::Type{Int64}(alignment::Any)::Any
549 end
```
The `.layout` field was getting inferred as `Any` which caused that
method to be invalidated when UnsafePointers.jl defined a new
`convert()` method.
Would it be possible to backport this to 1.12? UnsafePointers is a
dependency of PythonCall so it's quite widely used.
---------
Co-authored-by: Shuhei Kadowaki <40514306+aviatesk@users.noreply.github.com>
