add convenience accessors to is_dynamic_object_exprt

[kroening]

This commit adds two obvious convenience accessors to is_dynamic_object_exprt.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #7115 (0316056) into develop (8b7b7a9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #7115   +/-   ##
========================================
  Coverage    77.92%   77.92%           
========================================
  Files         1576     1576           
  Lines       181645   181645           
========================================
  Hits        141541   141541           
  Misses       40104    40104           

Impacted Files	Coverage Δ
src/util/pointer_expr.h	88.23% <ø> (ø)

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