[c++ interop] Swift should allow multiple SWIFT_CONFORMS_TO_PROTOCOL attributes on a C++ class.

(cherry picked from commit c05d1fc135)

include/swift/AST/DiagnosticsClangImporter.def

@@ -290,6 +290,9 @@ ERROR(both_returns_retained_returns_unretained, none,
       "SWIFT_RETURNS_UNRETAINED",
       (const clang::NamedDecl *))
 
+ERROR(redundant_conformance_protocol,none,
+     "redundant conformance of %0 to protocol '%1'", (Type, StringRef))
+
 ERROR(returns_retained_or_returns_unretained_for_non_cxx_frt_values, none,
       "%0 cannot be annotated with either SWIFT_RETURNS_RETAINED or "
       "SWIFT_RETURNS_UNRETAINED because it is not returning "