c_enum_typet::underlying_type() and c_bitfield_typet::underlying_type()

[kroening]

This introduces c_enum_typet::underlying_type() and c_bitfield_typet::underlying_type(), to be used instead of typet::subtype().

• 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 #6588 (6776f0c) into develop (907a214) will increase coverage by 0.01%.
The diff coverage is 93.20%.

@@             Coverage Diff             @@
##           develop    #6588      +/-   ##
===========================================
+ Coverage    76.19%   76.21%   +0.01%     
===========================================
  Files         1578     1578              
  Lines       181276   181393     +117     
===========================================
+ Hits        138132   138253     +121     
+ Misses       43144    43140       -4     

Impacted Files	Coverage Δ
jbmc/src/jbmc/jbmc_parse_options.cpp	72.31% <ø> (ø)
src/cbmc/cbmc_parse_options.cpp	77.39% <ø> (ø)
src/goto-analyzer/goto_analyzer_parse_options.cpp	71.94% <ø> (ø)
src/goto-cc/linker_script_merge.h	100.00% <ø> (ø)
src/goto-diff/goto_diff_parse_options.cpp	58.33% <ø> (ø)
...rc/goto-instrument/goto_instrument_parse_options.h	100.00% <ø> (ø)
src/goto-programs/goto_trace.cpp	80.73% <ø> (ø)
src/goto-programs/slice_global_inits.h	0.00% <ø> (ø)
src/solvers/smt2/smt2_conv.cpp	66.55% <0.00%> (+0.02%)	⬆️
src/ansi-c/c_typecast.cpp	80.48% <50.00%> (ø)
... and 44 more

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