[except.ctor] Retitle subclause as 'stack unwinding' (#7282)

The purpose of this subclause is to define stack unwinding,
which in specified in terms of the lifetime of objects, not
just class types.  Hence, while much of the text is addressing
interactions with contructors and destructors (the original
subclause title) it does more than just that.

Author: AlisdairM

source/exceptions.tex

@@ -359,7 +359,7 @@
 \end{note}
 
 
-\rSec1[except.ctor]{Constructors and destructors}%
+\rSec1[except.ctor]{Stack unwinding}%
 \indextext{exception handling!constructors and destructors}%
 \indextext{constructor!exception handling|see{exception handling, constructors and destructors}}%
 \indextext{destructor!exception handling|see{exception handling, constructors and destructors}}