Document the Clang policies on claiming support for a feature

We do not currently have this written down anywhere, and as a result,
we're sometimes inconsistent with how we handle feature test macros and
the feature status pages. This is an attempt to document what I
understand our existing policies to be instead of defining a new policy.

Note, this is being added to the Clang internals manual because we
don't have a separate document for Clang developer policies. At some
point, I think we may want a standalone document for that, and this
content can be moved there at that time.

Differential Revision: https://reviews.llvm.org/D146420