[flang] lower intrinsic constants to HLFIR

[flang] lower intrinsic constants to HLFIR

Use the utility to lower Constant<T> that was split from current lowering
in https://reviews.llvm.org/D136955.

The difference in HLFIR is the addition of a fir.declare on constant
outlined in memory so that all the information about them is available.

Lowering to HLFIR is enabled in Brideg::genExprValue to allow testing
of scalar constant lowering.

Differential Revision: https://reviews.llvm.org/D137084