[clang] Implement objc_non_runtime_protocol to remove protocol metadata

Summary:
Motivated by the new objc_direct attribute, this change adds a new
attribute that remotes metadata from Protocols that the programmer knows
isn't going to be used at runtime. We simply have the frontend skip
generating any protocol metadata entries (e.g. OBJC_CLASS_NAME,
_OBJC_$_PROTOCOL_INSTANCE_METHDOS, _OBJC_PROTOCOL, etc) for a protocol
marked with `__attribute__((objc_non_runtime_protocol))`.

There are a few APIs used to retrieve a protocol at runtime.
`@protocol(SomeProtocol)` will now error out of the requested protocol
is marked with attribute. `objc_getProtocol` will return `NULL` which
is consistent with the behavior of a non-existing protocol.

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75574