[flang] Update the conversion code for fir.coordinate_of

These are mostly small changes to make the code a bit clearer and more
consistent. Summary of changes:
  * add missing namespace qualifiers (that's the preference in Flang)
  * replace const member methods with static methods (to avoid passing
    the *this pointer unnecessarily)
  * rename `currentObjTy` (current object type) as `cpnTy` (component
    type) - the latter feels more fitting
  * remove redundant `return failure();` calls (` return
    mlir::emitError` gives the same result)
  * updated a few comments

Differential Revision: https://reviews.llvm.org/D122799