Fix Phobos dependencies on autodecoding

[Les De Ridder]

On Thursday, 15 August 2019 at 12:09:02 UTC, Vladimir Panteleev 
wrote:
> On Tuesday, 13 August 2019 at 07:08:03 UTC, Walter Bright wrote:
>> https://github.com/dlang/phobos/pull/7130
>
> [...]
>
> However, I noticed that one kind of the breakage is silent (the 
> code compiles and runs, but behaves differently). This makes me 
> uneasy, as it would be difficult to ensure that programs are 
> fully and correctly updated for a (hypothetical) transition to 
> no-autodecode.

I remembered this article from the wiki where you pointed this 
out back
in 2014:

https://wiki.dlang.org/Element_type_of_string_ranges

See also the forum thread that it links to.