ci: Build clang/sanitizers with libc++

[chfast]

Closes #906.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.29%. Comparing base (26d5609) to head (947a481).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #918   +/-   ##
=======================================
  Coverage   94.29%   94.29%           
=======================================
  Files         143      143           
  Lines       16138    16138           
=======================================
  Hits        15218    15218           
  Misses        920      920           

Flag	Coverage Δ
eof_execution_spec_tests	16.93% <ø> (ø)
ethereum_tests	26.90% <ø> (ø)
ethereum_tests_silkpre	18.68% <ø> (ø)
execution_spec_tests	17.91% <ø> (ø)
unittests	89.73% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.