[C++ Patch] Avoid duplicate warning

[Jason Merrill]

On 11/18/19 3:45 PM, Paolo Carlini wrote:

> functions like c_common_truthvalue_conversion are shared with the C 
> front-end thus don't get a tsubst_flags_t argument. It seems clear that 
> long term we have to do something about those but in the meanwhile we 
> have been using warning sentinels which, along the paths which certainly 
> must have all the warnings suppressed, work well for now and cannot 
> cause regressions. Here I propose to add (at least) one more to 
> ocp_convert since I have a straightforward testcase.

OK.

> Note sue if we want to proactively add, say, one for warn_parentheses too.

Not without a testcase.

Jason