Improve StringExp::length() to support transcoding
Handle extending string literal length to the target static array type

Both expression initializers and struct literal arguments should support implicit string lengthen.
fix Issue 13907 - Surrogate pairs in wchar string literal will cause incorrect length match

Stop considering lengthen conversion in StringExp::implicitConvTo().
Merge pull request #4222 from 9rnsr/fix13907

Issue 13907 - Surrogate pairs in wchar string literal will cause incorrect length match