[flang][hlfir] Fix c_null_ptr lowering in structure constructors (#68321)

Lowering handles C_PTR initial values that are designators or NULL()
inside structure constructors as an extension to support. This extension
is used by initial values generated for runtime derived type info.

But c_null_ptr wrongly fell into this extension path with HLFIR, causing
the initial value to be set to some (non null) address containing
c_null_ptr instead of c_null_ptr itself...

This was caused by the FIR lowering relying on genExtAddrInInitializer
to not place c_null_ptr inside an address. Fix this by only falling
through into the extension handling code if this is an extension: i.e,
the expression is some designated symbol or NULL().