[style] Remove trailing spaces
[Refactoring] Move StructLiteralExp::fill into StructDeclaration

Perhaps this is the most better design so it would be final move.
fix Issue 11510 - Relax restriction for overlapped pointer field access in safe code/during CTFE

Check overlapped field default initializations immediately after the struct size is determined.
fix Issue 11427 - anonymous unions break structs in @safe code
Merge pull request #2757 from 9rnsr/fix11510

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