[flang] Infrastructure improvements in utility routines

* IsArrayElement() needs another option to control whether it
  should ignore trailing component references.
* Add IsObjectPointer().
* Add const Scope& variants of IsFunction() and IsProcedure().
* Make TypeAndShape::Characterize() work with procedure bindings.
* Handle CHARACTER length in MeasureSizeInBytes().
* Fine-tune FindExternallyVisibleObject()'s handling of dummy arguments
  to conform with Fortran 2018: only INTENT(IN) and dummy pointers
  in pure functions signify; update two tests accordingly.

Also: resolve some stylistic inconsistencies and add a missing
"const" in the expression traversal template framework.

Differential Revision: https://reviews.llvm.org/D95011