Use generic exprt to avoid deprecated default constructor of symbol_exprt [blocks: #3768]

[tautschnig]

The implementation here is highly incomplete, in particular those symbols are
never set. If at some future date the implementation is completed then the type
of those members might be changed again.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a 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.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 0c566ef).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/98059524

[kroening]

Use optionalt instead?

[tautschnig]

Use optionalt instead?

Indeed much better, done.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 0ee3175).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/98090521