Exception handling in clang

[David Chisnall]

Hi Everyone,

I've just committed initial support for 'zero-cost' exception handling  
in clang.  Currently this only works for @finally and for @catch'ing  
id types (I plan on fixing that limitation soon).  I'd appreciate it  
if anyone who has code which uses exceptions could test this.  The  
support should currently be enough for anything using NS_DURING and  
friends to compile with zero-cost exceptions, rather than setjmp/ 
longjmp.

As always, please let me know if you find any cases where this is  
broken.

David