[Issue 5621] speller.c: implement type inference in speller suggestion

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=5621

--- Comment #4 from Iain Buclaw <ibuclaw at gdcproject.org> ---
It would still be a nice-to-have a type inference suggestion, instead of
"undefined identifier 'S'"

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