[asan] Allow enabling coverage at activation.

This is a re-commit of r224838 + r224839, previously reverted in r224850.
Test failures were likely (still can not reproduce) caused by two lit tests
using the same name for an intermediate build target.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@224853 91177308-0d34-0410-b5e6-96231b3b80d8