[Coverage] Use the most-recent completed region count (PR35437)

This is a fix for the coverage segment builder.

If multiple regions must be popped off the active stack at once, and
more than one of them end at the same location, emit a segment using the
count from the most-recent completed region.

Fixes PR35437, rdar://35760630

Testing: invoked llvm-cov on a stage2 build of clang, additional unit
tests, check-profile

llvm-svn: 319391