[phobos] win32 phobos build broken

[Brad Roberts]

One of the changes between 2015 and 2018 broke the win32 build:

http://d.puremagic.com/test-results/test_data.ghtml?dataid=3355

std\typecons.d(336): Error: cannot evaluate empty("") at compile time
std\typecons.d(371): Error: cannot evaluate injectNamedFields() at compile time
std\typecons.d(371): Error: argument to mixin must be a string, not
(injectNamedFields())