[OpenMP] Shutdown library on Windows if possible for better OMPT behavior

On Windows, child workers are terminated by the parent during the normal
program exit process (ExitProcess()) and they are not able to finish generating
their OpenMP events. We can force manual library shut down in __kmpc_end() to
fix this at least for the cases where __kmpc_end() is properly inserted.

Patch by Hansang Bae

Differential Revision: https://reviews.llvm.org/D52628

llvm-svn: 343619