@nospecialize for string_index_err (#57604)

The fields of `StringIndexError` are abstractly typed, so there's no
reason to specialize on a concrete type. The change seems like
it could prevent some invalidation on loading user code.

---------

Co-authored-by: Shuhei Kadowaki <40514306+aviatesk@users.noreply.github.com>

Author: nsajko

base/strings/string.jl

@@ -7,9 +7,9 @@ An error occurred when trying to access `str` at index `i` that is not valid.
 """
 struct StringIndexError <: Exception
     string::AbstractString
-    index::Integer
+    index::Int
 end
-@noinline string_index_err(s::AbstractString, i::Integer) =
+@noinline string_index_err((@nospecialize s::AbstractString), i::Integer) =
     throw(StringIndexError(s, Int(i)))
 function Base.showerror(io::IO, exc::StringIndexError)
     s = exc.string