Improve loplugin:doubleconvert

(The use of isa_and_nonnull<> instead of isa<> is necessary for cases like

  return (i_styleSettings.*i_getDefaultColor)();

in lcl_getEffectiveColor, svtools/source/table/gridtablerenderer.cxx.)

Change-Id: Iffc59b1146dd4ce13bbd3c8a6f46bd3c78a39344
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/109663
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

compilerplugins/clang/test/doubleconvert.cxx

@@ -17,6 +17,9 @@ int main()
     col2 = sal_Int32(
         col1); // expected-error@-1 {{redundant double conversion [loplugin:doubleconvert]}}
     (void)col2;
+    // expected-error@+1 {{redundant double conversion [loplugin:doubleconvert]}}
+    Color col3 = sal_Int32(Color());
+    (void)col3;
 }
 
 /* vim:set shiftwidth=4 softtabstop=4 expandtab cinoptions=b1,g0,N-s cinkeys+=0=break: */