[OpenMP] Target directive host codegen.

[OpenMP] Target directive host codegen.

This patch implements the outlining for offloading functions for code
annotated with the OpenMP target directive. It uses a temporary naming
of the outlined functions that will have to be updated later on once
target side codegen and registration of offloading libraries is
implemented - the naming needs to be made unique in the produced
library.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@249148 91177308-0d34-0410-b5e6-96231b3b80d8