SIL: make a linkage mismatch during de-serialization a human readable diagnostic

Although this error can only happen when tinkering with the stdlib's runtime functions, it's nice to get a readable error message. So far, the compiler just crashed later without a hint to the original problem.

include/swift/AST/DiagnosticsSIL.def

@@ -142,6 +142,10 @@ ERROR(deserialize_function_type_mismatch,Fatal,
       "type mismatch of function '%0', declared as %1 but used in a swift module as %2",
       (StringRef, Type, Type))
 
+ERROR(deserialize_function_linkage_mismatch,Fatal,
+      "linkage mismatch of function '%0', declared as %1 but expected as %2",
+      (StringRef, StringRef, StringRef))
+
 ERROR(without_actually_escaping_on_isolated_any,none,
       "withoutActuallyEscaping is currently unimplemented for '@isolated(any)' "
       "function values", ())