An `@__ctfe` function should do CTFE in a runtime context

[ntrel]

Instead of erroring:

int add(int a) @__ctfe => a + 2;

void main()
{
    int x = add(1); // currently errors, should work
}

@ibuclaw points out that would be equivalent to immediate functions
#22557 (comment)

[rikkimax]

According to mojo on Discord, the following code works:

int add(int a) @__ctfe {
    return a + 2;
}

[rikkimax]

Okay new message from Mojo who hasn't got access to GH atm, and says that this was wrong.