Result of CTFE should be implicitly castable to immutable

[Dmitry Olshansky]

My motivation is, of course, std.regex.

static immutable r = regex("abc");

This is perfectly fine but because in general there is some 
amount of impurtiy in regex and it won't work. Yet regex is 
called at CTFE so there is no way it can escape mutable pointer 
to its result.

Thoughts?