[style] fix indent
[Refactoring] Reorder code logic

Move "special cases" up in the function body
- In `AssignExp::toElem`, handling for the ref-init and NRVO.
- In `AssignExp::semantic`, handling for TOKconstruct case.
[Refactoring] Add CallExp::implicitCastTo
[Refactoring] Move construction handling in `AssingExp::semantic`
Merge pull request #2702 from 9rnsr/refactor_assign

[Refactoring] Gather assignment/construction handling into AssignExp