PR #9896: Opaque types for NVTX domain and string handles
PR #9896: Opaque types for NVTX domain and string handles

Imported from GitHub PR https://github.com/openxla/xla/pull/9896

This avoids having preprocessor checks on `GOOGLE_CUDA` inside some headers, which led to ODR violations.
Copybara import of the project:

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22e65458993aea1a9b441e6c043f7fe315ff4cc3 by Olli Lupton <olupton@nvidia.com>:

Opaque types for NVTX domain and string handles

This avoids having preprocessor checks on GOOGLE_CUDA inside some
headers, which led to ODR violations.

Merging this change closes #9896

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/openxla/xla/pull/9896 from olupton:opaque-nvtx 22e65458993aea1a9b441e6c043f7fe315ff4cc3
