With `system-headers-coverage=false`, functions defined in system
headers were not instrumented but corresponding covmaps were emitted. It
caused wasting covmap and profraw.
This change improves:
- Reduce object size (due to reduced covmap)
- Reduce size of profraw (uninstrumented system headers occupied
counters)
- Smarter view of coverage report. Stubs of uninstrumented system
headers will be no longer seen.
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