Add StructInitializer::fill to improve overlapped initializing check

- Right now `StructInitializer::semantic` retuens `ExpInitializer` with `StructLiteralExp`.
Update VarDeclaration::init by init->semantic result

The returned `Initializer` object may be different from `init`.
isEntirelyVoid should also work for StructLiteralExp
Fix TypeSArray::defaultInit for void static array type

Same as `TypeSArray::defaultInitLiteral`.
Right now StructInitializer::toDt is never called
Update StructLiteralExp::toDt for static array type field

StructLiteralExp::elements may hold the literal of the bottom type of static array field type.

struct S { int[4] sa; }   S s = S(1);
// (*sle->elements)[0]->op == TOKint64

This change is necessary to accommodate the case.
Update StructDeclaration::toDt

By using `StructInitializer::fill`, it could delegate the operation to `StructLiteralExp::toDt`.
Remove unused variables in StructInitializer
fix Issue 7727 - "static initializer" for non-static unions too
fix Issue 11132 - Odd diagnostic with C-style struct initializer when union field is present
fix Issue 11151 - Undetected overlapping initialization