[SEH] Implement GetExceptionCode in __except blocks

[SEH] Implement GetExceptionCode in __except blocks

This introduces an intrinsic called llvm.eh.exceptioncode. It is lowered
by copying the EAX value live into whatever basic block it is called
from. Obviously, this only works if you insert it late during codegen,
because otherwise mid-level passes might reschedule it.

llvm-svn: 235768