[PATCH] print notes only when -Wmismatched-tags succeeds (c++/96063)

[Jason Merrill]

On 7/6/20 8:21 PM, Martin Sebor wrote:
> The code for -Wmismatched-tags tests warn_mismatched_tags and when
> it's non-zero assumes that calls to warning_at() necessarily succeed,
> proceeding to call inform() to unconditionally issue informational
> notes.  This assumption is wrong when -Wmismatched-tags is enabled
> (and warn_mismatched_tags is nonzero) but the instance of the warning
> is for code in a system header and -Wsystem-headers is disabled.  In
> that case, the warning is suppressed but the notes are still printed.
> The attached near-trivial change makes the calls to inform() conditional
> on the result from warning_at().

OK.