Refactor expr2ct::convert_with_precedence to avoid MSVC limitation

[tautschnig]

Visual Studio fails with "fatal error C1061: compiler limit: blocks
nested too deeply" when attempting to add an additional expression to be
handled. To avoid this, factor out the trivial cases invoking
convert_function into a separate method.

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

[codecov]

Codecov Report

Merging #5912 (a4ef885) into develop (2459a7a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #5912   +/-   ##
========================================
  Coverage    75.01%   75.01%           
========================================
  Files         1431     1431           
  Lines       155609   155539   -70     
========================================
- Hits        116732   116684   -48     
+ Misses       38877    38855   -22     

Impacted Files	Coverage Δ
src/ansi-c/expr2c_class.h	100.00% <ø> (ø)
src/ansi-c/expr2c.cpp	68.62% <100.00%> (+<0.01%)	⬆️

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