Issue 11510 - Relax restriction for overlapped pointer field access in safe code/during CTFE

https://d.puremagic.com/issues/show_bug.cgi?id=11510

Implementing issue 11510 will also fix:
[Issue 11427](https://d.puremagic.com/issues/show_bug.cgi?id=11427) - anonymous unions break structs in @safe code.

And this is necessary compiler enhancement to fix 2.064 regression properly:
[Issue 11504](https://d.puremagic.com/issues/show_bug.cgi?id=11504) - [CTFE] JSONValue cannot make in CTFE