User-provided function definitions take precedence over built-ins

[tautschnig]

We should not replace calls to functions that have a body provided by
the user by our built-in definitions. While function names starting with
"_" are reserved in C and thus users shouldn't be using them, it is
certainly not true that anything named "nondet_*" is exclusively to be
captured by our built-ins.

• 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: c08a090).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/113333585