Make sure Core.Compiler can throw kwarg mismatch errors (#50174)

The _new_NamedTuple helper was in a Base-only branch, causing

```

julia> Core.eval(Core.Compiler, quote f(;a=1) = a end)
f (generic function with 1 method)

julia> Core.Compiler.f(;b=2)
ERROR: UndefVarError: `_new_NamedTuple` not defined
Stacktrace:
 [1] macro expansion
   @ Core.Compiler ./namedtuple.jl:0 [inlined]
 [2] structdiff(a::@NamedTuple{b::Int64}, b::Type{NamedTuple{(:a,)}})
   @ Core.Compiler ./namedtuple.jl:421
 [3] top-level scope
   @ REPL[2]:1
```

After this change, we have the expected
```
julia> Core.eval(Core.Compiler, quote f(;a=1) = a end)
f (generic function with 1 method)

julia> Core.Compiler.f(;b=2)
ERROR: MethodError: no method matching f(; b::Int64)

Closest candidates are:
  f(; a) got unsupported keyword argument "b"
   @ Core REPL[13]:1

Stacktrace:
 [1] kwerr(kw::@NamedTuple{b::Int64}, args::Function)
   @ Core.Compiler ./error.jl:165
 [2] top-level scope
   @ REPL[14]:1
```

base/namedtuple.jl

@@ -133,12 +133,6 @@ function NamedTuple{names, T}(nt::NamedTuple) where {names, T <: Tuple}
     end
 end
 
-# Like NamedTuple{names, T} as a constructor, but omits the additional
-# `convert` call, when the types are known to match the fields
-@eval function _new_NamedTuple(T::Type{NamedTuple{NTN, NTT}} where {NTN, NTT}, args::Tuple)
-    $(Expr(:splatnew, :T, :args))
-end
-
 function NamedTuple{names}(nt::NamedTuple) where {names}
     if @generated
         idx = Int[ fieldindex(nt, names[n]) for n in 1:length(names) ]
@@ -161,6 +155,12 @@ NamedTuple{names, Union{}}(itr::Tuple) where {names} = throw(MethodError(NamedTu
 
 end # if Base
 
+# Like NamedTuple{names, T} as a constructor, but omits the additional
+# `convert` call, when the types are known to match the fields
+@eval function _new_NamedTuple(T::Type{NamedTuple{NTN, NTT}} where {NTN, NTT}, args::Tuple)
+    $(Expr(:splatnew, :T, :args))
+end
+
 length(t::NamedTuple) = nfields(t)
 iterate(t::NamedTuple, iter=1) = iter > nfields(t) ? nothing : (getfield(t, iter), iter + 1)
 rest(t::NamedTuple) = t

test/compiler/AbstractInterpreter.jl

@@ -348,3 +348,8 @@ let NoinlineModule = Module()
         @test count(iscall((src, inlined_usually)), src.code) == 0
     end
 end
+
+# Make sure that Core.Compiler has enough NamedTuple infrastructure
+# to properly give error messages for basic kwargs...
+Core.eval(Core.Compiler, quote f(;a=1) = a end)
+@test_throws MethodError Core.Compiler.f(;b=2)