Drone CI: clang-p2996 reflection

[sdarwin]

Tests using the https://github.com/bloomberg/clang-p2996 compiler.
More information: https://github.com/cppalliance/drone-ci/blob/master/docs/reflection.md
Let me know if you encounter any problems.

[cppalliance-bot]

An automated preview of the documentation is available at https://1097.json.prtest2.cppalliance.org/libs/json/doc/html/index.html

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.70%. Comparing base (3c80c09) to head (78dc14d).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1097   +/-   ##
========================================
  Coverage    93.70%   93.70%           
========================================
  Files           91       91           
  Lines         9136     9136           
========================================
  Hits          8561     8561           
  Misses         575      575           

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[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/1097/pullrequest.html

[grisumbras]

Thanks!