[libc++] Force the ABI namespace to be a reserved identifier

Summary:
When the ABI namespace isn't a reserved identifier, we were issuing a
warning, but this should have been an error since the beginning. This
commit enforces that the ABI namespace is a reserved identifier, and
changes the ABI namespace used by LibFuzzer.

Reviewers: phosek, EricWF

Subscribers: mgorny, christof, jkorous, dexonsmith, #sanitizers, libcxx-commits, llvm-commits

Tags: #sanitizers, #libc, #llvm

Differential Revision: https://reviews.llvm.org/D69408