goto_inlinet isn't a messaget

[tautschnig]

Use a messaget member instead as there is no is-a relationship between
goto_inlinet and messaget.

• 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 #5594 (be06467) into develop (77a0496) will not change coverage.
The diff coverage is 72.22%.

@@           Coverage Diff            @@
##           develop    #5594   +/-   ##
========================================
  Coverage    69.32%   69.32%           
========================================
  Files         1242     1242           
  Lines       100418   100418           
========================================
  Hits         69619    69619           
  Misses       30799    30799           

Flag	Coverage Δ
cproversmt2	43.09% <ø> (ø)
regression	66.22% <72.22%> (ø)
unit	32.26% <50.00%> (ø)

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Impacted Files	Coverage Δ
src/goto-programs/goto_inline_class.cpp	82.40% <66.66%> (ø)
src/goto-programs/goto_inline_class.h	100.00% <100.00%> (ø)

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