pkg/coveragedb: store information about the covered functions #15312

Job	Run time
aux	3m 42s
build	4m 10s
arch (presubmit_arch_linux)	7m 29s
dashboard	2m 0s
old	6m 3s
arch (presubmit_arch_freebsd)	5m 29s
race	7m 7s
arch (presubmit_arch_netbsd)	3m 20s
race_dashboard	2m 36s
arch (presubmit_arch_openbsd)	3m 18s
gvisor	3m 5s
arch (presubmit_arch_darwin)	3m 15s
arch (presubmit_arch_windows)	3m 5s
arch (presubmit_arch_executor)	2m 53s
	57m 32s

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