[1.12] Fix codeinfo_for_const missing nargs and isva fields

[maleadt]

This PR back-ports a fix that was part of #59413 by @vtjnash to unbreak Compiler.inflate_ir! with const-return IR:

julia> _const_return_func(@nospecialize(x)) = 42
_const_return_func (generic function with 1 method)

julia> mi = Compiler.specialize_method(only(methods(_const_return_func)), Tuple{typeof(_const_return_func), Int}, Core.svec())

MethodInstance for _const_return_func(::Int64)

julia> worlds = Compiler.WorldRange(Base.get_world_counter(), Base.get_world_counter())
Compiler.WorldRange(0x000000000000987a, 0x000000000000987a)

julia> ci = Compiler.codeinfo_for_const(Compiler.NativeInterpreter(), mi, worlds, Core.svec(), 42)
CodeInfo(
1 ─     return 42
)

julia> ir = Compiler.inflate_ir!(ci, mi)
ERROR: AssertionError: invalid `given_argtypes` for `mi`

AFAIU this is because of codeinfo_for_const not setting nargs and isva fields on the returned CodeInfo object:

julia> ci.nargs = 2
2

julia> ir = Compiler.inflate_ir!(ci, mi)
 1 ─     return 42                                                                                                                                         │

[maleadt]

CI failure (Distributed hang on i686-linux) already present on backports branch, e.g., https://buildkite.com/julialang/julia-release-1-dot-12/builds/525#019bdca2-f16c-42da-9a45-e7dbb0375727.