Emit the Itanium LSDA for unknown EH personalities on Win64

This fixes lots of generic CodeGen tests that use __gcc_personality_v0.
This suggests that using ExceptionHandling::MSVC was a mistake, and we
should instead classify each function by personality function. This
would, for example, allow us to LTO a binary containing uses of SEH and
Itanium EH.

llvm-svn: 226019