Introduce floatbv_rounding_mode(unsigned)

[kroening]

This introduces a function to produce the constant expression that is
consumed by the floatbv_* expressions as the rounding mode.

Using this function in ieee_floatt removes the dependency on c_types.h.

• 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.

[tautschnig]

Shouldn't the comment also clarify that the type needs to be the same as that of the __CPROVER_rounding_mode symbol (which, actually, seems to be a signed int)? Edit: no, it doesn't matter, float_utilst::set_rounding_mode will create constants of the appropriate size.

[tautschnig]

I'd prefer using an integer of minimal size of a fixed-32-bits one, but likely this won't matter much.

[tautschnig]

@kroening I believe tests would pass after a rebase.

[codecov]

Codecov Report

Merging #6288 (bd9520c) into develop (8065a69) will decrease coverage by 0.00%.
The diff coverage is 90.90%.

@@             Coverage Diff             @@
##           develop    #6288      +/-   ##
===========================================
- Coverage    75.97%   75.97%   -0.01%     
===========================================
  Files         1508     1509       +1     
  Lines       163380   163381       +1     
===========================================
- Hits        124131   124130       -1     
- Misses       39249    39251       +2     

Impacted Files	Coverage Δ
...ensitivity/variable_sensitivity_dependence_graph.h	79.31% <ø> (ø)
src/util/floatbv_expr.cpp	0.00% <0.00%> (ø)
src/util/floatbv_expr.h	73.58% <ø> (ø)
src/util/ieee_float.cpp	88.01% <0.00%> (ø)
src/util/ieee_float.h	85.71% <ø> (ø)
...nsitivity/three_way_merge_abstract_interpreter.cpp	93.02% <91.17%> (ø)
src/analyses/ai.cpp	76.12% <91.66%> (ø)
src/analyses/ai.h	57.36% <100.00%> (ø)
...sensitivity/three_way_merge_abstract_interpreter.h	100.00% <100.00%> (ø)
...sitivity/variable_sensitivity_dependence_graph.cpp	84.58% <100.00%> (ø)
... and 12 more

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