Introduce ansi-c/c_expr.h

[kroening]

This introduces a new header file, ansi-c/c_expr.h, which is to collect
expressions that are specific to and internal to the C frontend.

• 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/
• n/a 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 #5923 (7dec92e) into develop (2459a7a) will increase coverage by 0.00%.
The diff coverage is n/a.

❗ Current head 7dec92e differs from pull request most recent head 13ddc84. Consider uploading reports for the commit 13ddc84 to get more accurate results

@@           Coverage Diff            @@
##           develop    #5923   +/-   ##
========================================
  Coverage    75.01%   75.02%           
========================================
  Files         1431     1431           
  Lines       155609   155563   -46     
========================================
- Hits        116732   116708   -24     
+ Misses       38877    38855   -22     

Impacted Files	Coverage Δ
src/util/expr.h	97.84% <0.00%> (ø)
unit/util/expr.cpp	100.00% <0.00%> (ø)
src/goto-symex/goto_state.h	100.00% <0.00%> (ø)
src/ansi-c/expr2c.cpp	68.62% <0.00%> (+<0.01%)	⬆️
src/util/expr.cpp	76.47% <0.00%> (+0.71%)	⬆️

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[tautschnig]

Trying to figure out why check-cpplint isn't failing here, as it should. Working on this in #5929.

[tautschnig]

I'm not (yet) convinced: a further (assembler) and one stronger (goto-programs) dependency on ansi-c doesn't seem right.

[tautschnig]

src/assembler/module_dependencies.txt does not currently list ansi-c, so we're introducing a new dependency on ansi-c here. And, really, this means that the expression is not limited to the C front-end.