Location coverage reporting should ignore globals

[tautschnig]

We iterate over instructions in non-built-in functions, and it is
therefore reasonable to expect that all source locations have functions
that they belong to. If symbols declared at global scope are part of
expressions then this shouldn't yield calling them out in location
coverage. (That's just where they were declared, it doesn't even
strictly imply their assignments were covered.)

Fixes: #6978

• 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 #6981 (fffc77e) into develop (257bfc4) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #6981   +/-   ##
========================================
  Coverage    77.83%   77.84%           
========================================
  Files         1571     1571           
  Lines       180484   180487    +3     
========================================
+ Hits        140488   140508   +20     
+ Misses       39996    39979   -17     

Impacted Files	Coverage Δ
src/goto-instrument/cover_basic_blocks.cpp	89.56% <100.00%> (+0.27%)	⬆️
src/util/simplify_expr.cpp	84.28% <0.00%> (+1.20%)	⬆️

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