[flang] add ExtendedValue type helpers and factory::genZeroValue

Add some helpers to get the base type and element type of
fir::ExtendedValue and to test if a fir::ExtendedValue is
a derived type with length parameters.

Add a new helper factory::genZeroValue to generate zero scalar value for
all the numerical types and false for logicals.

These helpers are used only in lowering for now, so add unit tests.

Differential Revision: https://reviews.llvm.org/D118795