@nospecialize for string_index_err and StringIndexError

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

Author: nsajko

base/strings/string.jl

@@ -8,8 +8,16 @@ An error occurred when trying to access `str` at index `i` that is not valid.
 struct StringIndexError <: Exception
     string::AbstractString
     index::Integer
+    function StringIndexError((@nospecialize s::AbstractString), (@nospecialize i::Integer))
+        new(s, i)
+    end
+end
+function StringIndexError(str, ind)
+    s = convert(AbstractString, str)
+    i = convert(Integer, ind)
+    StringIndexError(s, i)
 end
-@noinline string_index_err(s::AbstractString, i::Integer) =
+@noinline string_index_err((@nospecialize s::AbstractString), (@nospecialize i::Integer)) =
     throw(StringIndexError(s, Int(i)))
 function Base.showerror(io::IO, exc::StringIndexError)
     s = exc.string