Use llvm-symbolizer's JSON output to provide function start lines

[insilications]

When analyzing a perf.data profile converted automatically via perf_to_profile via pprof -raw perf.data, no function start lines (s=0) are present in any of the locations. With 813a5fb, this can be easily solved by using the same JSON frame data from llvm-symbolizer to provide StartLine for Function.start_line. This solves #823.

[aalexand]

Thanks for adding this. Would it be possible to add a test that exercises the new code?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.01%. Comparing base (0ed6a68) to head (4f09060).
Report is 34 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #884      +/-   ##
==========================================
+ Coverage   66.86%   67.01%   +0.14%     
==========================================
  Files          44       44              
  Lines        9824     9800      -24     
==========================================
- Hits         6569     6567       -2     
+ Misses       2794     2780      -14     
+ Partials      461      453       -8     

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