Track and print the number of tests that were discovered but not
executed due to test selection options:
* --filter (regex filter)
* --max-tests (limits number of tests)
* sharding feature
With this change all discovered tests are accounted for: every
discovered test is included in one of the counts printed in the summary.
Reviewed By: jdenny
Differential Revision: https://reviews.llvm.org/D78078