[flang][NFC] add genType(FunctionRef<T>) entry points in lowering

This will help lowering to HLFIR to not use the AsGenericExpr/AsExpr
patterns that copies sub-expresssions into evaluate::SomeExpr so that
they can be passed to helpers. Sub-expressions like FunctionRef can
be heavy (hundreds of arguments, constant array expression arguments...).

Differential Revision: https://reviews.llvm.org/D138997