Annotate types with ABSL attributes?

[jiawen]

Abseil has "portable" (ifdefed around clang) attributes for annotating:

• ABSL_ATTRIBUTE_VIEW that something is a view (e.g., array_ref)
• ABSL_ATTRIBUTE_OWNER that something is owns all the data (e.g., array)
• ABSL_ATTRIBUTE_LIFETIME_BOUND that a parameter passed into a free or member function needs to outlive the function.

Any objection to adding these in guarded behind some ifdefs of our own?