clang-tidy: Silence use-equals-default warning

clang-tidy 7 has an option to suppress this warning in macros which
defaults to '1'.  Set the option to '0' and silence the warning
explicitly.

.clang-tidy

@@ -34,4 +34,6 @@ HeaderFilterRegex: 'Source/cm[^/]*\.(h|hxx|cxx)$'
 CheckOptions:
   - key:   modernize-use-default-member-init.UseAssignment
     value: '1'
+  - key:   modernize-use-equals-default.IgnoreMacros
+    value: '0'
 ...