unsafe const fn declaration order

[brson]

The recent implementation uses the syntax unsafe const fn, but the RFC says const unsafe fn.

[thepowersgang]

Actually, the RFC text says unsafe const fn, but a comment on the PR says const unsafe fn

[nikomatsakis]

Discussed in the lang meeting today and decided we might as well stick to our decision, arbitrary though it may have been, and thus we should accept only const unsafe fn. I think that most of us would prefer to actually accept modifiers in any order, but it seems weird to accept just these two particular modifiers in any order, and thus we should just open a more general RFC about allowing arbitrary modifiers to appear in any order.

[thepowersgang]

I'm doing a final compilation check on the commit above, once it's done I'll file a PR reversing the order.

[thepowersgang]

RFC update PR rust-lang/rfcs#1335 and implementation update #29274