[WebAssembly] Add exception handling option and feature

Summary:
Add a llc command line option and WebAssembly architecture feature for
exception handling.

Reviewers: dschuff

Subscribers: jfb, sbc100, jgravelle-google, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D43683

llvm-svn: 326004
diff --git a/llvm/lib/CodeGen/TargetPassConfig.cpp b/llvm/lib/CodeGen/TargetPassConfig.cpp
index 9628363..bfe8f15 100644
--- a/llvm/lib/CodeGen/TargetPassConfig.cpp
+++ b/llvm/lib/CodeGen/TargetPassConfig.cpp
@@ -653,6 +653,9 @@
     addPass(createWinEHPass());
     addPass(createDwarfEHPass());
     break;
+  case ExceptionHandling::Wasm:
+    // TODO to prevent warning
+    break;
   case ExceptionHandling::None:
     addPass(createLowerInvokePass());