initial support for ObjC exceptions with the GNU runtime:

"This patch is a first pass at adding support for exceptions for the GNU runtime.  There are a few limitations at present:

- @synchronized() is not yet supported at all.  gcc currently emits calls to runtime library functions that don't exist for this directive.

- Only id @catch statements are currently working.  This is enough for NS_DURING and friends, but I need to spend more time reading the output from gcc -S to work out how it finds the class pointer to make arbitrary class type catch statements work.

- I've tested it with a few common cases[1] and the clang test suite (which doesn't test exceptions for the GNU runtime, but shows I haven't broken anything else), but there are probably a lot of cases I've missed."

Patch by David Chisnall!

llvm-svn: 71198