Fix compilation failure on DMD/Phobos master

[mihails-strasuns]

Because of range-ification of some Phobos functions
implicit convertion of DirEntry to string does not
work anymore.

Looks like this won't be considered regression, thus submitting the fixup.

[mihails-strasuns]

(dlang/phobos#3261 (comment))

[s-ludwig]

Interesting. I wonder if alias this shouldn't cause the range predicate of globMatch to match anyway...

[mihails-strasuns]

I think the way isNarrowString!T and friends are defined right now, alias this is completely ignored. Not sure about it but feels most likely explanation

[s-ludwig]

Just hat a look and isForwardRange is the offender: It checks if is(typeof(T.save) == T). To make alias this work, it would have to do something weaker, such as is(T : typeof(T.save)).