[OpenMP] Fix initializer not working on AMDGPU

The RAII class used for debugging RTL entry used a shared variable to
keep track of the current depth. This used a global initializer, which
isn't supported on AMDGPU. This patch removes the initializer and
instead sets it to zero when the state is initialized in the runtime.

Reviewed By: jdoerfert, JonChesterfield

Differential Revision: https://reviews.llvm.org/D113963