[Diagnostics] Reword malformed swiftsourceinfo diagnostic

The original wording is not helpful to a user if they don't know what a
swiftsourceinfo actually is. Change the wording to provide some more
context, ie. that it is an unstable format and should not be
distributed.

Resolves rdar://80785874

include/swift/AST/DiagnosticsSema.def

@@ -812,7 +812,9 @@ ERROR(serialization_allowing_error_type,none,
       "allowing deserialization of error type '%0' in module '%1'",
       (StringRef, StringRef))
 WARNING(serialization_malformed_sourceinfo,none,
-      "unable to use malformed module source info '%0'", (StringRef))
+      "'%0' is either malformed or generated by a different Swift version. "
+      "Note that it uses an unstable format and may leak internal project "
+      "details, it should not be distributed alongside modules", (StringRef))
 
 ERROR(reserved_member_name,none,
       "type member must not be named %0, since it would conflict with the"

test/Serialization/load-invalid-sourceinfo.swift

@@ -14,4 +14,4 @@
 // RUN: echo -n 'abcde' > %t/empty.swiftsourceinfo
 // RUN: %target-swift-frontend -typecheck -I %t %s -verify
 
-import empty // expected-warning{{unable to use malformed module source info}}
+import empty // expected-warning{{is either malformed or generated by a different Swift version}}