Add UNIMPLEMENTED_FEATURE macro

[thomasspriggs]

So that messages can be included to describe exactly what it is which is unimplemented.

Split from code review comment on #6131 for separate review.

• 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.
• 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).
• My commit message includes data points confirming performance improvements (if claimed). None claimed.
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6152 (590fa37) into develop (437420d) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #6152      +/-   ##
===========================================
- Coverage    74.95%   74.88%   -0.07%     
===========================================
  Files         1447     1448       +1     
  Lines       158197   158202       +5     
===========================================
- Hits        118580   118477     -103     
- Misses       39617    39725     +108     

Impacted Files	Coverage Δ
src/util/invariant.h	82.92% <ø> (ø)
src/solvers/flattening/bv_pointers.cpp	82.66% <100.00%> (-0.04%)	⬇️
unit/util/invariant.cpp	100.00% <100.00%> (ø)
src/ansi-c/literals/convert_float_literal.cpp	73.80% <0.00%> (-19.05%)	⬇️
src/ansi-c/c_typecheck_code.cpp	66.26% <0.00%> (-11.66%)	⬇️
src/ansi-c/c_storage_spec.cpp	86.84% <0.00%> (-10.53%)	⬇️
src/util/pointer_offset_size.cpp	86.94% <0.00%> (-5.23%)	⬇️
src/util/std_types.cpp	83.95% <0.00%> (-4.94%)	⬇️
src/util/std_code.h	91.50% <0.00%> (-3.23%)	⬇️
src/util/simplify_expr_floatbv.cpp	94.57% <0.00%> (-1.56%)	⬇️
... and 1 more

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[tautschnig]

Should be util/invariant.h to make builds happy.