Rename identifiers named `__output`

Summary:
In the CHERI clang compiler __output and __input are keywords and therefore
we can't compile libc++ with our compiler.

Reviewers: mclow.lists, EricWF, theraven

Reviewed By: EricWF

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D39537

llvm-svn: 318144