Issue 3449 - const and invariant struct members do not behave according to spec

https://issues.dlang.org/show_bug.cgi?id=3449

It's time to remove 'implicit static' declaration!

First PR: #93
Warning: #2076
Deprecation: #3038
I'd rather keep this line, and rewrite it to be:
```
error(loc, "%s field with initializer must be static, __gshared, or an enum",
```
because it also gives what corrective action to take.