[OpenMP] Change debugging symbol to weak_odr linkage

The new device runtime uses an internal variable to set debugging. This
variable was originally privately linked because every module will have
a copy of it. This caused problems with merging the device bitcode
library because it would get renamed and there was not a way to refer to
an external, private symbol. This changes the symbol to weak_odr so it
can be defined multiply, but will not be renamed.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D109997