[PATCH] CodeGen support for x86_64 SEH catch handlers in LLVM

[Reid Kleckner]

Hi rjmccall,

This adds ExceptionHandling::MSVC, used by the x86_64-pc-windows-msvc
triple. It assumes that filter functions have already been outlined in
either the frontend or the backend. Filter functions are used in place
of the landingpad catch clause type info operands. In catch clause
order, the first filter to return true will catch the exception.

The C specific handler table expects the landing pad to be split into
one block per handler, but LLVM IR uses a single landing pad for all
possible unwind actions. This patch papers over the mismatch by
synthesizing single instruction BBs for every catch clause to fill in
the EH selector that the landing pad block expects.

Missing functionality:
- Accessing data in the parent frame from outlined filters
- Cleanups (from __finally) are unsupported, as they will require
  outlining and parent frame access
- Filter clauses are unsupported, as there's no clear analogue in SEH

In other words, this is the minimal set of changes needed to write IR to
catch arbitrary exceptions and resume normal execution.

http://reviews.llvm.org/D6300

Files:
  include/llvm/CodeGen/MachineModuleInfo.h
  include/llvm/MC/MCAsmInfo.h
  lib/CodeGen/AsmPrinter/AsmPrinter.cpp
  lib/CodeGen/AsmPrinter/EHStreamer.cpp
  lib/CodeGen/AsmPrinter/EHStreamer.h
  lib/CodeGen/AsmPrinter/Win64Exception.cpp
  lib/CodeGen/AsmPrinter/Win64Exception.h
  lib/CodeGen/MachineModuleInfo.cpp
  lib/CodeGen/Passes.cpp
  lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
  lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h
  lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
  lib/Target/X86/MCTargetDesc/X86MCAsmInfo.cpp
  test/CodeGen/X86/seh-basic.ll
  test/CodeGen/X86/seh-safe-div.ll
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