Revert "[Coverage] Precise region termination with deferred regions"

This reverts commit r310010. I don't think there's anything wrong with
this commit, but it's causing clang to generate output that llvm-cov
doesn't do a good job with and the fix isn't immediately clear.

See Eli's comment in D36250 for more context.

I'm reverting the clang change so the coverage bot can revert back to
producing sensible output, and to give myself some time to investigate
what went wrong in llvm.

llvm-svn: 310154