Add markup for libc++ dylib availability
Libc++ is used as a system library on macOS and iOS (amongst others). In order
for users to be able to compile a binary that is intended to be deployed to an
older version of the platform, clang provides the
availability attribute https://clang.llvm.org/docs/AttributeReference.html#availability_
that can be placed on declarations to describe the lifecycle of a symbol in the
See docs/DesignDocs/AvailabilityMarkup.rst for more information.
Differential Revision: https://reviews.llvm.org/D31739