[Coverage] Do not write out coverage mappings with zero entries

After r275121, we stopped mapping regions from system headers. Lambdas
declared in regions belonging to system headers started producing empty
coverage mappings, since the files corresponding to their spelling locs
were being ignored.

The coverage reader doesn't know what to do with these empty mappings.
This commit makes sure that we don't produce them and adds a test. I'll
make the reader stricter in a follow-up commit.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@276716 91177308-0d34-0410-b5e6-96231b3b80d8