Register the flush function for each compile unit.

Register the flush function for each compile unit.

For each compile unit, we want to register a function that will flush that
compile unit. Otherwise, __gcov_flush() would only flush the counters within the
current compile unit, and not any outside of it.

PR15191 & <rdar://problem/13167507>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177340 91177308-0d34-0410-b5e6-96231b3b80d8