rename RETURN -> SET_RETURN_VALUE

[kroening]

This 1) renames the enum constant RETURN to SET_RETURN_VALUE and 2) renames
the methods associated with instructions of this type accordingly.

The goal of this change is to prevent misunderstandings of the semantics of
SET_RETURN_VALUE instructions, which do not alter control flow.

• 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/
• n/a 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 #6250 (42253c9) into develop (ccfbaee) will increase coverage by 0.03%.
The diff coverage is 65.06%.

❗ Current head 42253c9 differs from pull request most recent head 55ab09b. Consider uploading reports for the commit 55ab09b to get more accurate results

@@             Coverage Diff             @@
##           develop    #6250      +/-   ##
===========================================
+ Coverage    76.18%   76.22%   +0.03%     
===========================================
  Files         1484     1482       -2     
  Lines       162092   161868     -224     
===========================================
- Hits        123495   123387     -108     
+ Misses       38597    38481     -116     

Impacted Files	Coverage Δ
src/analyses/custom_bitvector_analysis.cpp	54.97% <0.00%> (ø)
src/analyses/escape_analysis.cpp	64.15% <0.00%> (ø)
src/analyses/global_may_alias.cpp	0.00% <0.00%> (ø)
src/analyses/interval_domain.cpp	59.03% <0.00%> (ø)
src/analyses/local_bitvector_analysis.cpp	75.14% <ø> (ø)
src/analyses/local_may_alias.cpp	61.18% <ø> (ø)
src/analyses/local_safe_pointers.cpp	81.55% <ø> (ø)
src/analyses/uncaught_exceptions_analysis.cpp	92.77% <ø> (ø)
...riable-sensitivity/variable_sensitivity_domain.cpp	87.57% <0.00%> (ø)
src/goto-instrument/dot.cpp	0.00% <0.00%> (ø)
... and 35 more

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