Avoid several bad show_default invalidations (JuliaLang#37125)

This fixes ~50 quite critical invalidations from loading StaticArrays,
which manages to hit the package-loading code, the docstring code,
and the `MethodError` display code. Since particularly the first two
are used by *everything*, these invalidations are about as bad as it
gets given that this is not a huge number of invalidations (at least, by
our old standards).

The invalidation of the package-loading code is due to a rather
hilarious chain of events.
[This method definition](https://github.com/JuliaArrays/StaticArrays.jl/blob/ad583c99768f3a381ba20b1a0782c9ca50890b50/src/SArray.jl#L125-L126)
ends up invalidating [`show_default`](https://github.com/JuliaLang/julia/blob/89d6b469b633dde8f92b4d245ca3d8e5606e229c/base/show.jl#L417),
which got called because `stale_cachefile` used an outdated variable
name in a [`@debug` macro](https://github.com/JuliaLang/julia/blob/89d6b469b633dde8f92b4d245ca3d8e5606e229c/base/loading.jl#L1518)
that ended up grabbing the global `mod` function. Since
`show(::IO, ::Function)` can call `show_default`, this ended up
invalidating basically all the package-loading code.

The docstring code got invalidated because
`show(::IO, ::Pair{Symbol,Any})`
[calls `show_default`](https://github.com/JuliaLang/julia/blob/89d6b469b633dde8f92b4d245ca3d8e5606e229c/base/show.jl#L907).
This PR provides a dedicated path that circumvents `show_default`.

Finally, an innocuous-seeming function-print in
`showerror(::IO, ::MethodError)` also led to quite a chain of invalidation.

* Specify that getaddrinfo(::AbstractString) returns a `Union{IPv4,IPv6}`

These are the only two types supported by `Sockets`, so this seems
reasonable, although it does block further extension.

base/errorshow.jl

@@ -287,7 +287,8 @@ function showerror(io::IO, ex::MethodError)
             )
             varnames = ("scalar", "array")
             first, second = arg_types_param[1] <: Number ? varnames : reverse(varnames)
-            print(io, "\nFor element-wise $(nouns[f]), use broadcasting with dot syntax: $first .$f $second")
+            fstring = f === Base.:+ ? "+" : "-"  # avoid depending on show_default for functions (invalidation)
+            print(io, "\nFor element-wise $(nouns[f]), use broadcasting with dot syntax: $first .$fstring $second")
         end
     end
     if ft <: AbstractArray

base/loading.jl

@@ -1515,7 +1515,7 @@ function stale_cachefile(modpath::String, cachefile::String)
                     skip_timecheck = true
                     break
                 end
-                @debug "Rejecting cache file $cachefile because it provides the wrong uuid (got $build_id) for $mod (want $req_build_id)"
+                @debug "Rejecting cache file $cachefile because it provides the wrong uuid (got $build_id) for $req_key (want $req_build_id)"
                 return true # cachefile doesn't provide the required version of the dependency
             end
         end

base/show.jl

@@ -927,7 +927,7 @@ function gettypeinfos(io::IO, p::Pair)
 end
 
 function show(io::IO, p::Pair)
-    isdelimited(io, p) && return show_default(io, p)
+    isdelimited(io, p) && return show_pairtyped(io, p)
     typeinfos = gettypeinfos(io, p)
     for i = (1, 2)
         io_i = IOContext(io, :typeinfo => typeinfos[i])
@@ -938,6 +938,11 @@ function show(io::IO, p::Pair)
     end
 end
 
+function show_pairtyped(io::IO, p::Pair{K,V}) where {K,V}
+    show(io, typeof(p))
+    show(io, (p.first, p.second))
+end
+
 function show(io::IO, m::Module)
     if is_root_module(m)
         print(io, nameof(m))
@@ -2424,6 +2429,7 @@ end
 
 const undef_ref_str = "#undef"
 
+show(io::IO, ::UndefInitializer) = print(io, "UndefInitializer()")
 
 """
     summary(io::IO, x)

stdlib/Sockets/src/addrinfo.jl

@@ -140,7 +140,7 @@ getaddrinfo(host::AbstractString, T::Type{<:IPAddr}) = getaddrinfo(String(host),
 function getaddrinfo(host::AbstractString)
     addrs = getalladdrinfo(String(host))
     if !isempty(addrs)
-        return addrs[begin]
+        return addrs[begin]::Union{IPv4,IPv6}
     end
     throw(DNSError(host, UV_EAI_NONAME))
 end