increase verbosity of warning about missing __builtin_ declaration

[kroening]

The __builtin_ declarations of gcc and clang are highly version-specific, and constantly changing. User see a lot of warnings about missing declarations of __builtin_ functions owing to compiler header files that use these builtins in inline wrappers. This increases the verbosity level of these warnings to "debug".

• 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).
• 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

All modified and coverable lines are covered by tests ✅

Project coverage is 78.37%. Comparing base (8646bfa) to head (c427d7b).
Report is 4 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8333   +/-   ##
========================================
  Coverage    78.37%   78.37%           
========================================
  Files         1726     1726           
  Lines       188322   188325    +3     
  Branches     18460    18456    -4     
========================================
+ Hits        147606   147609    +3     
  Misses       40716    40716           

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