unwindset: Also warn about loops in functions without body

[tautschnig]

f14cd09 demoted some failures to warnings. This commit does so for
another case: if no body for the function is available, issue a warning
rather than failing an exception.

This situation will typically arise when using an unwindset
specification originally meant for CBMC with goto-instrument, where
library functions might not yet be present.

• 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.
• 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 #6524 (5f33908) into develop (bb59fa6) will increase coverage by 0.00%.
The diff coverage is 93.44%.

@@           Coverage Diff            @@
##           develop    #6524   +/-   ##
========================================
  Coverage    75.98%   75.98%           
========================================
  Files         1576     1578    +2     
  Lines       180850   180910   +60     
========================================
+ Hits        137411   137467   +56     
- Misses       43439    43443    +4     

Impacted Files	Coverage Δ
src/goto-instrument/unwindset.cpp	85.00% <33.33%> (-3.43%)	⬇️
src/goto-programs/remove_returns.cpp	98.15% <100.00%> (+0.02%)	⬆️
...smt2_incremental/construct_value_expr_from_smt.cpp	100.00% <100.00%> (ø)
...smt2_incremental/construct_value_expr_from_smt.cpp	100.00% <100.00%> (ø)

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