Model it as throwing so that the exception can be caught.

This is generally not expected to have significant code-size
impact because the contents of the @autoreleasepool block
are very likely to contain a call, very likely at the same
cleanup level as the @autoreleasepool itself.

rdar://13660038

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179630 91177308-0d34-0410-b5e6-96231b3b80d8