[flang] Lower addresses inside global initializers in HLFIR

Move the code to lower an expression to address or a box in HLFIR from
Bridge.cpp to ConvertExpr.cpp so that it can be used inside
ConvertVariable.cpp (that needs to use a different symbol map that the
one held in the bridge).

Lower NULL to hlfir.null.

This allows lowering derived type constant structure constructors with
pointer components into fir.global.

Differential Revision: https://reviews.llvm.org/D141276