[flang] Implement F2023 TYPEOF and CLASSOF type specifiers

The Fortran 2023 Standard extends declaration-type-spec with two new type specifiers:

TYPEOF ( data-ref )
CLASSOF ( data-ref )

These allow declarations to indirectly use the type of a previously-defined entity. CLASSOF additionally implies polymorphism.
More details in - https://github.com/llvm/llvm-project/blob/main/flang/docs/F202X.md#typeof-and-classof
This is marked as N in https://releases.llvm.org/21.1.0/tools/flang/docs/FortranStandardsSupport.html#fortran-2023