Issue 9665 - Structure constant members can not be initialized if have opAssign

http://d.puremagic.com/issues/show_bug.cgi?id=9665

Issue 9665 is a root cause of some newly appeared regressions from 2.061-2.063.

[Issue 11257](http://d.puremagic.com/issues/show_bug.cgi?id=11257) - Allow whole implicit conversion if one or more overlapped field could
This is necessary for `std.typecons.Rebindable`, and `std.datetime.Systime` that's using `Rebindable`.

Regressions which will be fixed:
[Issue 11186](http://d.puremagic.com/issues/show_bug.cgi?id=11186) - Regression (2.061): Presence of Variant and const field invokes opAssign
[Issue 11246](http://d.puremagic.com/issues/show_bug.cgi?id=11246) - [REG 2.063] Struct initialized in constructor is destroyed first
[Issue 10357](http://d.puremagic.com/issues/show_bug.cgi?id=10357) - std.typecons.Nullable!(SysTime).Nullable.__ctor!() error instantiating