Merge pull request #39717 from nkcsgexi/83450549-5.5

Author: DougGregor

include/swift/AST/DiagnosticsModuleDiffer.def

@@ -88,7 +88,7 @@ ERROR(not_inheriting_convenience_inits,none,"%0 no longer inherits convenience i
 
 ERROR(enum_case_added,none,"%0 has been added as a new enum case", (StringRef))
 
-ERROR(demangled_name_changed,none,"%0 has mangled name changing from '%1' to '%2'", (StringRef, StringRef, StringRef))
+WARNING(demangled_name_changed,none,"%0 has mangled name changing from '%1' to '%2'", (StringRef, StringRef, StringRef))
 
 WARNING(cannot_read_allowlist,none,"cannot read breakage allowlist at '%0'", (StringRef))