Cleanup cpp_convert_typet: no abbreviations, pre-increment, size_t

[tautschnig]

Expand _cnt suffix to _count (as per coding standard), use
pre-increment, use std::size_t instead of unsigned as there could be
arbitrarily many occurrences.

• 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]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: dfa220c).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/108853055
Status will be re-evaluated on next push.
Common spurious failures include: the cbmc commit has disappeared in the mean time (e.g. in a force-push); the author is not in the list of contributors (e.g. first-time contributors); compatibility was already broken by an earlier merge.

[tautschnig]

Closing as #4558 takes a completely different approach, effectively removing most of this code.