rename index_type and enum_constant_type

[kroening]

The index type used for arrays and the type of enum constants are
language-specific. This renames the functions index_type and
enum_constant_type in c_types.h to c_index_type and c_enum_constant_type,
respectively, to clarify this.

• 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.

[codecov]

Codecov Report

Merging #6577 (4271a0a) into develop (5ff4932) will decrease coverage by 0.00%.
The diff coverage is 85.84%.

@@             Coverage Diff             @@
##           develop    #6577      +/-   ##
===========================================
- Coverage    76.01%   76.01%   -0.01%     
===========================================
  Files         1579     1579              
  Lines       181304   181286      -18     
===========================================
- Hits        137817   137796      -21     
- Misses       43487    43490       +3     

Impacted Files	Coverage Δ
src/analyses/local_may_alias.cpp	63.80% <0.00%> (ø)
src/cpp/cpp_typecheck_constructor.cpp	89.00% <ø> (ø)
src/cpp/cpp_typecheck_initializer.cpp	29.26% <0.00%> (ø)
src/goto-instrument/function.cpp	0.00% <0.00%> (ø)
src/goto-programs/graphml_witness.cpp	53.37% <0.00%> (+0.14%)	⬆️
src/util/c_types.h	100.00% <ø> (ø)
src/goto-programs/string_instrumentation.cpp	25.29% <25.00%> (-0.36%)	⬇️
src/util/c_types.cpp	89.18% <50.00%> (-0.87%)	⬇️
src/util/std_types.cpp	85.71% <50.00%> (ø)
src/goto-symex/symex_other.cpp	84.61% <66.66%> (ø)
... and 35 more

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