[OpenMP] Replace device kernel linkage with weak_odr

Currently the device kernels all have weak linkage to prevent linkage
errors on multiple defintions. However, this prevents some optimizations
from adequately analyzing them because of the nature of weak linkage.
This patch replaces the weak linkage with weak_odr linkage so we can
statically assert that multiple declarations of the same kernel will
have the same definition.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D122443