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 <[email protected]>

Author: JamesWrigley

base/runtime_internals.jl

@@ -580,8 +580,9 @@ alignment of the elements, not the whole object.
 """
 function datatype_alignment(dt::DataType)
     @_foldable_meta
-    dt.layout == C_NULL && throw(UndefRefError())
-    alignment = unsafe_load(convert(Ptr{DataTypeLayout}, dt.layout)).alignment
+    layout = dt.layout::Ptr{Cvoid}
+    layout == C_NULL && throw(UndefRefError())
+    alignment = unsafe_load(convert(Ptr{DataTypeLayout}, layout)).alignment
     return Int(alignment)
 end
 
@@ -664,8 +665,9 @@ Return the number of pointers in the layout of a datatype.
 """
 function datatype_npointers(dt::DataType)
     @_foldable_meta
-    dt.layout == C_NULL && throw(UndefRefError())
-    return unsafe_load(convert(Ptr{DataTypeLayout}, dt.layout)).npointers
+    layout = dt.layout::Ptr{Cvoid}
+    layout == C_NULL && throw(UndefRefError())
+    return unsafe_load(convert(Ptr{DataTypeLayout}, layout)).npointers
 end
 
 """