Simply create_alias_expression in code_contracts.cpp

[feliperodri]

CBMC version: >5.15
Operating system: Ubuntu 18.04 or macOS Mojave 10.14.6
Exact command line resulting in the issue: N/A.
What behaviour did you expect: N/A.
What happened instead:

1. Adds a Doxygen documentation;
2. The implementation of create_alias_expression can be simplified by using and object of type exprt::operandst with each call to same_object doing operands.push_back(same_object(leftPtr, rightPtr); and then doing return disjunction(operands); at the very end.

[feliperodri]

Fixed in #5538