[WinEH] Generate .xdata for catch handlers

[WinEH] Generate .xdata for catch handlers

This lets us catch exceptions in simple cases.

N.B. Things that do not work include (but are not limited to):
- Throwing from within a catch handler.
- Catching an object with a named catch parameter.
- 'CatchHigh' is fictitious, we aren't sure of its purpose.
- We aren't entirely efficient with regards to the number of EH states
  that we generate.
- IP-to-State tables are sensitive to the order of emission.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233767 91177308-0d34-0410-b5e6-96231b3b80d8