Add -Wduplicate-protocol for existing diagnostic

Expose a warning flag for warn_duplicate_protocol_def. This allows control
over the severity of duplicate protocol definitions.

For example -Werror=duplicate-protocol or
#pragma clang diagnostic ignored "-Wduplicate-protocol".

Patch provided by Dave Lee!

Differential Revision: https://reviews.llvm.org/D26406

llvm-svn: 286487