First cut at zero-cost EH support.

First cut at zero-cost EH support.
 - Still manually generates the EH code; the parts related to cleanup
   need to be integrated into the cleanup stack (for proper
   interaction with VLAs, etc.).

 - Some differences vs gcc in corner cases; I believe our behavior is
   correct but need to verify/file bugs vs gcc.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@65809 91177308-0d34-0410-b5e6-96231b3b80d8