[Typed throws] Workaround for LLVM miscompile of unused swifterror, add executable test

lib/IRGen/IRGenSIL.cpp

@@ -3775,6 +3775,13 @@ void IRGenSILFunction::visitFullApplySite(FullApplySite site) {
       errorDest.phis[0]->addIncoming(errorValue, Builder.GetInsertBlock());
     } else {
       Builder.emitBlock(typedErrorLoadBB);
+
+      // Create a dummy use of 'errorValue' in the catch BB to workaround an
+      // LLVM miscompile that ends up taking the wrong branch if there are no
+      // uses of 'errorValue' in the catch block.
+      // FIXME: Remove this when the following radar is fixed: rdar://116636601
+      Builder.CreatePtrToInt(errorValue, IGM.IntPtrTy);
+
       auto &ti = cast<LoadableTypeInfo>(IGM.getTypeInfo(errorType));
       Explosion errorValue;
       ti.loadAsTake(*this, getCalleeTypedErrorResultSlot(errorType), errorValue);

test/IRGen/typed_throws_exec.swift

@@ -0,0 +1,29 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift -module-name=test -enable-experimental-feature TypedThrows %s -o %t/a.out
+// RUN: %target-run %t/a.out | %FileCheck %s
+// REQUIRES: executable_test
+
+public enum MyError : Error {
+  case a
+  case b
+}
+
+public func throwing() throws(MyError) -> Int {
+  throw MyError.a
+}
+
+func wat() {
+  fatalError("this cannot happen")
+}
+
+public func catching() {
+  do {
+    try throwing()
+    wat()
+  } catch {
+  }
+}
+
+catching()
+print("Works!")
+// CHECK: Works!