Tolerate missing summaries for call graphs

[hermanventer]

Description

The root cause of the #1190 issue is that there is no MIR for CStr::from_bytes_with_nul and no foreign contract either. That means that the analysis of main is necessarily imprecise and analyzing the rest of the function will just generate a bunch of diagnostics that are probably false positives. Providing a foreign contract for CStr::from_bytes_with_nul will make the problem go way for the specific repro case, but won't solve the problem for all code until the vast majority of functions without MIR have been provided with contracts.

That is not going to happen any time soon, so the alternative is to embrace the fact that call graphs are already so abstract that no one gets too excited when a non reachable call (false positive) shows up in the call graph. As #1190 illustrates, false negatives are much of more of a concern here. Consequently, the analysis will now carry on when even when it is incomplete and prone to false positives, if the analysis is done in order to retrieve a call graph.

Fixes #1190

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• API change with a documentation update
• Additional test coverage
• Code cleanup or just keeping up with the latest Rustc nightly

How Has This Been Tested?

./validate.sh
Checked that the repro in #1190 now works as expected.

[codecov]

Codecov Report

Merging #1191 (61d0d27) into main (2368aac) will increase coverage by 0%.
The diff coverage is 66%.

@@          Coverage Diff          @@
##            main   #1191   +/-   ##
=====================================
  Coverage     76%     76%           
=====================================
  Files         23      23           
  Lines      16460   16462    +2     
=====================================
+ Hits       12623   12626    +3     
+ Misses      3837    3836    -1     

Impacted Files	Coverage Δ
checker/src/call_visitor.rs	81% <50%> (-1%)	⬇️
checker/src/block_visitor.rs	75% <100%> (ø)
checker/src/abstract_value.rs	78% <0%> (+<1%)	⬆️

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