[Flang] Get fir::SequenceType from hlfir::ExprType before getShape. (#90055)

This PR is to fix issue #88889 . 
Because the type of an actual argument of an array expression of
character has type of `hlfir::ExprType`, we need to transform it to
`fir::SequenceType` before calling `getBoxTypeWithNewShape`.
Calling `hlfir::ExprType::getShape` inside of `getBoxTypeWithNewShape`
will introduce a circular dependency on FIRDialect and HLFIRDialect
libraries.