Autoharness: Harness Generation Improvements

[carolynzech]

The important changes are:

• 37db951, which will allow autoharness to eventually work with modifies clauses. Since we don't generate autoharnesses for functions that take pointers as arguments anyway, this doesn't affect current functionality.
• a325fd8, which ensures that the --function passed to CBMC is the mangled name of the automatic harness intrinsic, not the target function

Towards #3832

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.