Type-assert DataType.layout in some Base functions

[JamesWrigley]

This prevents a bunch of invalidations, specifically these ones in Julia 1.12 with UnsafePointers.jl (CC @cjdoris):

 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:

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.

[JamesWrigley]

I believe the test failure is fixed by #59879.